Major Pat Hibbs says because there is no ash in the area, there is no threat to health.
Poor weather conditions have obscured a direct view of the active vent, but GNS Science says white steam clouds have been observed at the historically active Te Mari craters area.
Monitoring continues of the ash spread from last night's eruption and of the mountain in case of any further activity.
Ash is being blown east.
Civil Defence says it has received no reports of injuries or damage caused by the eruption.
New Zealand Post said there will be no deliveries by posties today in Napier, Hastings and Waipukurau.
Spokesperson Jaimee Burke said New Zealand Post had heeded Civil Defence advice for people to avoid being outdoors due to ash and haze caused by the eruption.
"We regret any inconvenience to customers but the safety and wellbeing of our Posties who are out on foot and bikes is a priority.
"We are keeping a close eye on conditions and will resume deliveries once conditions allow," Burke said.
Hungry animals grazing short pastures are particularly at risk, it says.
Any concerned dairy farmers should feed good quality feed supplement, shift stock to long pasture, and provide plenty of uncontaminated water.
Both the skifields on Mount Ruapehu are open this morning, with operators saying the eruption of Mt Tongariro poses no threat to the Whakapapa and Turoa ski areas.
The volcanic vents on Tongariro, Ngauruhoe, and Ruapehu were independent of each other and there was no reason or indication of volcanic activity on Ruapehu, a Mt Ruapehu spokesman said.
"The Te Mari crater where the eruption occurred is at least 20km away from the ski area."
The ash was blowing to the east, away from the skifields.
Air New Zealand said flights operating to airports in Gisborne, Rotorua, Taupo, Napier and Palmerston North could be delayed or cancelled.
A spokeswoman could not say how many flights had already been disrupted this morning.
Air New Zealand general manager of airline operations and safety David Morgan said the airline was working with the relevant authorities to safely make adjustments to flight routes to ensure aircraft remained clear of any ash.
Passengers were advised to continue checking the Air New Zealand website for updates.
"As I was coming up from Waiouru.....I saw this beautiful, big cloud and I thought 'gee that looks like a volcanic plume'. Just as I thought that there was a great big orange flash," Rodda said.
He said he saw a thick cloud of ash develop and fine grey ash started falling.
''(The cloud) looked like a fist, basically, at an angle across the sky. About the wrist section of the fist there was an orange ball of flash I could see.
"It was quite impressive."
There were a few startled comments over the radio from other truck drivers in the area, he said.
Auckland University volcanologist Phil Shane said the small eruption this morning could signal the start of a pattern of explosions at the central North Island mountain.
He drew a comparison to the Caribbean volcano Montserrat, which rumbled to life in 1995 and has continued to erupt until today.
A more violent eruption in the coming days or a drop off in all volcanic activity was also possible, Associate Professor Shane said.
"We don't know how long it could go. It could be a one off or it could go weeks, months years or even a decade."
A fly-over the mountain confirmed it was a steam plume only and there was no discolouration of the plume.
This indicated that there was no change in volcanic ash or gas levels.
"We were unable to see the impact immediately around the craters so we still can't confirm just which crater the eruptions occurred from,'' he said.
"We've had a small-scale volcanic eruption. It appears to be driven in the hydrothermal rather than the magmatic process, there's been an ash plume, there's been ash-fall down wind.''
The weather was at its best this morning to conduct a fly-over, and it was now working against scientists to get up in a helicopter to observe the mountain.
Similarly, there had not been any action since the eruption.
Changes in volcanic gas levels had been recorded in the past few weeks, but there was not any significant increase leading into the eruption.
A flyover of the mountain this morning revealed a steam plume coming from the mountain but there was no indication of volcanic ash this morning.
Scientists were unable to see the immediate impact of the eruption on the mountain because of bad weather and low cloud.
Mr Scott described it as a "small scale eruption''.
It was a hypothermal-type eruption.
"I wouldn't be surprised if there was some small hypothermal-driven eruptions."
In Gisborne in the Hexton area, a Kawatere Road resident was outside on the way to his shed to turn on a hot water cylinder when the volcano erupted hundreds of kilometres away.
He had been watching sport on television and went to the shed to turn on the cylinder in preparation to home-kill two pigs today.
"The pheasants started crowing away and I thought an earthquake was coming next. They always do that when there is going to be an earthquake.
"Then the dogs started howling and I thought it might be a big one. I heard three massive explosions. It was boom, boom, boom, and then it sounded like a stock rolling flat-out down a hill. It was unbelievable.''
When he got into bed his wife said she had woken to the sound of rattling windows.
"I told her it sounded like the South Island had just dropped off.'"
A few other people reported waking to the sound of rattling windows thinking an earthquake had just happened.
The Tongariro Crossing and its four huts were currently closed.
At this stage, the department's facilities around Tongariro were affected by the eruption, but the ski fields on Mt Ruapehu were not affected and were open for operation.
A layer of ash approximately 1mm in depth has come to ground inland from Napier and northeast into the Wairoa area. There is a noticeable sulphuric smell in the air around Napier-Hastings, advised by Taradale and Havelock North people.
Civil Defence said air quality reporting also showed increased levels of PM10 particulate matter readings at all three air monitoring stations in Awatoto, Marewa Park (Napier) and St Johns (Hastings).
Hawkes Bay CDEM group controller Ian Macdonald asked schools, childcare centres and businesses to use commonsense, given the current clear skies. Unless ash was falling it was safe to be outside, he said.
Police closed State Highway 1 and State Highway 46 as a precautionary matter overnight but they had since reopened.
The police focus remained on public safety.
Three Department of Conservation huts on Mt Tongariro were currently being cleared.
All locals residents were urged to remain calm and check water supplied to make sure they were not contaminated.
Pommie74 said: "It stinks of sulphur here in Napier and we have ash on the car."
The volcanic vents on Tongariro, Ngauruhoe, and Ruapehu were independent of each other and there was no reason or indication of volcanic activity on Ruapehu, a Mt Ruapehu spokesman said.
"The Te Mari crater where the eruption occurred is at least 20km away from the ski area."
The ash was blowing to the east, away from the skifields.
- APNZ
"I've spoken to my regional representatives around central North Island and at this stage the ash falling doesn't seem to be a major problem.
"Information on the GNS website says the ash is about 20,000 feet so we are hoping we can sneak under that cloud, and the ash blow out to sea."
Mr Wills, from Te Pohue, said he had phoned family at his northern Hawke's Bay property this morning to see if there was any ash visible.
"They say they're ok. But we are watching the situation carefully because if there is a likelihood of a lot of ash falling on pasture in areas, that could mean some animal health issues for farmers."
"It's just carpeted everything, all the fields, cars, trees - the whole landscape looks quite murky and grey," he said.
"Roofs in this area, they're all absolutely coated in this ash."
Contractors are currently sweeping the ash from State Highway 46, Mr Morton said.
"There is also quite a noticeable smell ... quite a murky smell. We've had reports in Waiouru that there is a sulphur-like smell in the area. That is not the case here."
Weather conditions about the Central Plateau are overcast, and the summit of Tongariro is not visible, he said.
Mr MacDonald said people in the affected areas should stay indoors and limit their driving. Do not use your windscreen wipers if there is ash on your window.
He said people on tank water supply should disconnect their water tank from the roof, and hose down their roof before reconnecting.
Mr MacDonald said people should be prepared for possible power outages.
Captain David Morgan, Air New Zealand general manager airline operations and safety and chief pilot, said the airline is working with the relevant authorities to make adjustments to flight routes to ensure aircraft remain clear of any ash.
"We will not fly through ash and are constantly taking guidance from the CAA and the MetService to ensure we can continue to carry passengers where safe routes and altitudes are available."
Passengers are advised to check the Air New Zealand website for flight arrivals and departures information.
"The advice to people is to stay indoors, because volcanic ash can obviously be a health hazard, if they're indoors please close windows and doors to try and limit the entry of ash."
Mr Cholewa says at this stage not all areas alerted are affected by ash, but that situation could change.
"We're working actively with GNS Science who operate the monitoring equipment on the mountains, and with police so all the information from the ground is being gathered, and decisions will be based on that information.
"Evacuations have not been ordered, please listen to the radio for advice from local authorities and police, any evacuations would be issued at that level, and based on the evidence from GNS Science."
He said the ash has not drifted to Ruapehu, south of Tongariro.
"The ash is the concern - if the wind changes, which we don't believe it will."
7.01am - Mt Tongariro eruption: Where will the ash go?
6.40am - GNS volcanologist Brad Scott told Radio New Zealand the eruption began from the Te Mare craters at the north end of Mt Tongariro at 11.50pm.
An earthquake lasting about five minutes accompanied the eruption and residents reportedly heard the explosions.
"When they went outside to have a look they saw the volcano starting to erupt. They saw incandescent blocks - glowing hot blocks - and they saw an eruption column being developed and within about five minutes they were experiencing a light local ash fall."
GNS had been aware of some issues at Mt Tongariro for a few weeks, "but to be honest we didn't see anything in the latest data up until last night that indicated it was ready to erupt", Mr Scott said.
He said there was likely to be further activity.
"There's not showing any escalation - the earthquake activity hasn't increased or anything like that - but we would probably anticipate some more activity now that the craters have broken through."
6.35am - The aviation colour code has been raised to red.
6.34am - Weatherwatch.co.nz say the wind direction overnight was a westerly and throughout today the winds will turn more north westerly as a front and large low approaches.
"The winds don't look especially strong over the next few days as the centre of a low crosses the North Island - the lighter the winds are the more ash will fall locally around the mountain and less likely to cause widespread disruptions further afield," says head weather analyst Philip Duncan.
5:43am - The Civil Aviation Authority's Meterology Manager Peter Lechner says there will be a new advisory issued to aircraft just after 6am. He said the ash cloud was affecting a zone of airspace stretching as far as from Tongariro to north of Gisborne then south to Hawkes Bay and possibly northern Wairarapa. The CAA alerted all aircraft using a volcanic ash advisory system, working with MetService. Mr Lechner said that ash can build up in the turbines of aeroplanes and helicopters, causing engines to stall.
"It can result in significant flight risk."
5:41am - A media conference in relation to the eruption on Mount Tongariro will be held at 9.30am this morning at the Taupo police station.
5:39am - Clayton Bolt, a passing motorist told RadioLive that he saw a massive white cloud coming from the side of Mt Tongariro. "I put my foot down. I said, I'm going."
5:00am - A truck driver has told Radio New Zealand that the ash cloud has caused thick dust and reduced visibility on the Desert Road. Bryn Rodda said he saw a large cloud rising from the mountain with orange flashes. http://www.radionz.co.nz/news/national/112587/tongariro-erupts,-threat-warning-issued
4:00am - Aviation colour code increased from orange to red. Potential threat remains for Waikato, Hawkes Bay, Gisborne, Manawatu-Wanganui, Bay of Plenty and Taranaki regions.
3:00am - Ash fall reported as far east as Napier CBD.
2:00am - Police close Desert Rd due to possible danger to the public. SH47, 4 and SH5 remain open. Police are sending search and rescue teams up Mt Tongariro at first light to check no one is stranded in huts. No reports of injuries or damage. A joint agency incident management centre is established at the Whakapapa Department of Conservation visitor centre.
1:45am - National advisory from Civil Defense - people in Waikato, Hawkes Bay, Gisborne, Manawatu-Wanganui, Bay of Plenty and Taranaki regions advised to take precautions by staying indoors and avoiding ash fall.
1:00am - residents reported to have left their houses on the southern shores of Lake Rotoaira.
12:25am - GNS Science Volcanic Alert Bulletin issued. Volcanic alert level raised from level 1 to level 2. Aviation colour code changed from yellow to orange.
12:00am - reports of loud explosions, lightning and plumes of smoke.
11:50pm Monday - Eruption occurs, ash fall recorded in local vicinity.
- Herald Online and APNZ
