Images of the violent volcanic eruption near Tonga are "hugely concerning", Prime Minister Jacinda Ardern says.
In a social media post Ardern said communication had been difficult following the eruption of the underground volcano Hunga Tonga-Hunga Haʻapai last night.
The eruption sent ash, steam and gas up to 20km into the air and has caused untold damage in the Pacific nation.
"[Our] defence force team and Ministry of Foreign Affairs are working as we speak to establish what's needed and how we can help," the post from Ardern said.
Little is known yet about the impacts on Tonga itself as power is out and communication lines are down.
Meanwhile, in New Zealand the National Emergency Management Agency (Nema) has extended the warning for strong and unusual currents and unpredictable surges.
"The National Advisory: Tsunami Activity issued following the large eruption in Tonga is cancelled.
"The advice from GNS Science, based on ocean observations, is that the beach and marine threat has now passed for all areas.
"Strong and unusual currents and unpredictable surges will continue in places for up to another 24 hours in some locations around the entire country."
Nema is advising people near the coast to remain vigilant and take extra precautions.
Labour MP for Panmure-Otahuhu Jenny Salesa said on Sunday evening she has been informed no lives were lost on the main island of Ha'apai.
Last night locals posted video of a tsunami flooding the coastline, surging through houses and a church. "Pray for us," wrote one Tongan resident on social media, as people fled to higher ground amid panicked screaming. The sky turned black as the cloud from the eruption covered the sun, and much of the country is thought to be covered in ash.
A Royal New Zealand Air Force P-3K Orion is set to undertake a reconnaissance flight from RNZAF Base Auckland Whenuapai early on Monday morning.
The crews will assist in an initial impact assessment of the area near Tonga and low-lying islands.
"The flight departure time is dependent on the ash cloud dissipating enough to enable safe flying in the area," a New Zealand Defence Force spokesperson said.
The plane is set to leave Whenuapai at 8am, however that is subject to change.
The Ministry of Foreign Affairs and Trade (Mfat) said the New Zealand High Commission in Nuku'alofa was monitoring the situation closely and was in contact with local authorities.
"As yet there are no official reports of injuries or deaths. However, communications are limited," the statement from an Mfat spokesperson said.
"Damage assessments are under way and New Zealand has formally offered to provide assistance to Tonga. An NZDF P3 Orion is on stand-by to provide aerial surveillance as soon as atmospheric conditions allow."
Mfat had made an initial $500,000 available to respond to any requests from the Tongan government as they came in.
Thirty New Zealanders were registered on the SafeTravel website as being in Tonga. Mfat advised all Kiwis in Tonga to register their details on www.safetravel.govt.nz, and to follow the advice of local authorities.
Telecommunications company 2degrees has joined Spark in offering to waive all fees for those trying to phone friends and family in Tonga.
The company announced on social media late this afternoon that it is waiving costs for landline and mobile voice calls and texts between Tonga and New Zealand until midnight on Sunday, January 23.
Save the Children says it is ready to respond in Tonga and the surrounding islands affected by tsunamis and tidal waves.
"The immediate concern in Tonga is for air and water safety due to ash and smoke. The government has asked the public to wear masks and use bottled water for now," the charity says.
Authorities had reported no casualties at this stage and all Save the Children staff and volunteers located in Tonga, Fiji and Vanuatu were safe and accounted for.
Save the Children Fiji CEO Shairana Ali said the organisation was ready to help their Tonga brothers and sisters. Save the Children has a small presence in Tonga but more staff and resources in Fiji and Vanuatu.
Both those nations had seen tsunami and tidal waves, causing some damage to coastal areas but no fatalities.
"Communications have been affected but we're doing what we can to ensure those living in low-lying coastal areas are moving to higher ground.
"The experts have warned that volcanic activity may continue, causing new tsunami warnings to be issued, and recommended people stay indoors to avoid the ash and smoke."
"The people of Pacific Island nations are sadly becoming very used to facing disasters. They are incredibly resilient communities."
Tsunami alerts around the Pacific after eruption
The eruption has sent shock waves around the world and put many countries around the Pacific on high alert for tsunamis or surges, including in New Zealand.
Fresh warnings have been issued this morning by Civil Defence, which says people on the north and east coast of the North Island, the west coast of the South Island and the Chatham Islands could all likely see strong and unusual currents and unpredictable surges at the shore.
Tutukaka Marina and some campgrounds in Northland were evacuated overnight due to a tsunami surge that caused "significant damage", sinking a number of boats and badly damaging others.
The marina has been evacuated this morning as more big waves may be on their way, officials say.
Many boats at Tutukaka were "completely destroyed" by a wave around 2m higher than the high tide line - believed to be caused by a combination of Cyclone Cody and the huge volcanic eruption in Tonga.
Tutukaka locals were at the marina this morning attempting to clean up the extensive damage, but they have now been asked to leave.
While no further tsunami warning has been issued, officials are asking everyone who is not needed to leave the space as they are aware of big waves coming in.
They say the evacuation is a precautionary measure - they do not want more people in the area to add to the hassle.
Elsewhere beaches along New Zealand's coastline have been closed by lifeguards, including at Whangamatā on the Coromandel Peninsula.
A message posted online by lifeguards said the beach had been closed because of the very rough sea and swell conditions, made worse by the volcanic activity out of Tonga.
"This all adds up to the beach and water being potentially very dangerous. Our advice is to stay out of the water and off the beach."
Civil Defence is warning of further strong surges in coming days, saying "locations that have previously been calm can suddenly experience unpredictable surges".
• Live: 'Pray for us' - surging waves hit Tonga following eruption
• Tsunami warning for Tonga as underwater volcano Hunga-Tonga-Ha'apai erupts
• Hunga-Tonga-Ha'apai: Surging waves hit island nation
• Cyclone Cody expected to brush past New Zealand late Sunday
"Strong currents and surges can injure and drown people," the latest Civil Defence alert says.
"There is a danger to swimmers, surfers, people fishing, small boats and anyone in or near the water close to shore.
"People in or near the sea should move out of the water, off beaches and shore areas and away from harbours, rivers and estuaries," it says.
Flooding of coastal land areas was not expected.
Last night Civil Defence Northland issued a tsunami warning for people living along the coastline following the violent eruption of underground volcano Hunga Tonga-Hunga Haʻapai. It warned of "strong and unusual currents and unpredictable surges at the shore".
But it appears some people were still caught unawares last night. Pictures from Tutukaka Marina show boats that had slipped their moorings, with visible damage to their hulls.
Civil Defence is warning sightseers to stay away as the hazardous surges are continuing, and are likely along the entire east coast of Northland.
Local man Tim Alexander said his boat was among about 30 that had been destroyed, although it was hard to tell at this stage.
His own boat was a complete loss, he said. The wave had cleared the breakwater which was about 2 metres higher than the high tide line.
He had several friends who lived on their boats but all were safe. "We're very thankful - nobody was on their boat, they heard it coming in. From what I understand it was a big event, they got out really fast."
His biggest concern was that there was no warning from Civil Defence ahead of the surge, with no phone alerts or tsunami siren. He had seen the general warning put out on the news but had taken little notice given there was no tsunami alert in the area.
"We've got a tsunami alert system here in Tutukaka but we had no warning whatsoever.
"There have been multiple alarms in the last 24 months and we've had to evacuate but there's been no damage to the marina, and almost [no] waves - this time there was no notification and it's completely destroyed the entire marina."
He estimated damage to both the marina and the boats was in the millions of dollars.
Just before 9am people were tending to their property as well as securing rubbish to stop it floating out to sea. Alexander said the surges seemed to have subsided this morning - although Civil Defence has warned fresh waves could come without warning.
Around 10.30am a reporter at the scene said officials were evacuating the area for health and safety reasons, and asking people to leave the marina if they were not strictly needed.
It's not yet clear what those health and safety concerns are, as there is no fresh tsunami warning - however the marina jetties are extensively damaged.
A number of people on social media are also reporting they did not get a civil defence alert to their phones prior to the surge.
On Saturday evening water was sucked out of half the bay followed by "massive surges" up into the estuary, Rebecca Hendl-Smith tweeted. "Pretty scary and I reckon most of us on the Sandspit evacuated soon after that."
Police said they had attended a number of tsunami-related call-outs in the Far North, with the first call coming in at 11.20pm from Te Rere.
"Police, fire and Coast Guard assisted in evacuating some boats from [Tutukaka] marina after large waves surged ashore. The waves have also damaged some pontoons and boats moored at the marina," a spokeswoman said.
She was unsure how many people had been evacuated.
Civil Defence had also advised of a campground being flooded at Mahinepua Bay. Police had attended the campsite to assist with evacuations, she said.
Civil Defence believes the surge was caused by a combination of Cyclone Cody and the Tonga eruption.
Just before midnight Civil Defence Northland said it was aware of "a number of impacts" that had taken place along Northland's east coast.
"These impacts have generally been localised and either in, or close to, the water, but in at least one instance have caused significant damage: Tutukaka Marina, where there has been damage to a number of boats and marina structures, with people who live on their boats within the marina evacuated as a precaution."
This morning Civil Defence said daylight confirmed the extent of the damage, and it was "sad news" for a number of boat owners.
Frequent and strong surge activity was continuing this morning and was likely to be happening along the whole of Northland's east coast.
"Experience from past tsunami events has been that this activity can continue for a number of days, and that locations that have previously been calm can suddenly experience unpredictable surges.
"Please continue to take extreme care in and around the water and don't give in to the temptation to go sightseeing - the combination of the effects of Cyclone Cody and already-heightened sea levels with tsunami surge from the Tonga eruption has the potential to create hazards that have not previously been experienced."
Civil Defence said last night a number of coastal campgrounds had also been evacuated or campers relocated because of incoming waves.
"It's a challenge to distinguish the effects of the storm surge generated by Cyclone Cody, from currents/surges resulting from the volcanic eruption in Tonga, but our best assessment at this stage is that the two combined in a number of specific places, with local landforms also playing a part," Civil Defence said.
"Damage assessment/cleanup efforts will begin in earnest in the morning at Tutukaka Marina in particular (noting that Tonga is also facing a major cleanup)."
People were warned to take great care around beachfronts and estuaries as past tsunamis had caused unpredictable effects for a number of days, while Cyclone Cody was also still causing large waves.
"It's not a time for taking chances".
Civil Defence thanked everyone who had helped last night, including emergency services, community members, iwi, and businesses.