Police had received no reports of injury or damage, but the Ministry of Civil Defence & Emergency Management advised people in affected areas to stay indoors.
There have been no mandatory evacuations ordered by authorities, but some residents have chosen to leave the area.
A ``potential threat to New Zealand advisory'' is in place for Waikato, Hawkes Bay, Gisborne, Manawatu-Wanganui, Bay of Plenty and Taranaki.
State Highway 1 between Rangipo and Waiouru (the Desert Road) and SH46 west of Rangipo were closed due to possible danger to motorists.
SH1 was reopened before 8am today but SH46 remained closed.
Air New Zealand said regional flights to and from destinations east of Tongariro could be delayed or cancelled and passengers should check its website for updates.
A joint agency incident management centre has been established at the Whakapapa Department of Conservation Visitor Centre.
GNS Science are monitoring the seismic activity on the mountain and Department of Conservation and Police staff are checking tramping huts at first light.
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 yesterday.
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.''
Mt Tongariro last erupted between 1896 and 1897.
Truck driver Bryn Rodda was one of the first to see the eruption.
``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,'' he told National Radio.
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.''
Weatherwatch said where the ash would go depended on the wind.
``We've updated our wind predictions - made more difficult by the fact a large low from the Tasman Sea is expected to cross over Central Plateau. The winds around the centre of the low are very light - which is good news for any further potential ash clouds, as it would decrease the chances of significant drift,'' the forecaster said in an update.
Civil defence spokesman Vince Cholewa told NewstalkZB ash could reach those living in Waikato, Hawkes Bay, Gisborne, Manawatu-Wanganui, Bay of Plenty and Taranaki.
``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 not all areas alerted were 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.''b
