There are no official reports of injuries or deaths in Tonga, as the Defence Force prepares to head to the islands, Prime Minister Jacinda Ardern says.
Communication with the islands has been cut off since yesterday evening and members of the Tongan community in New Zealand are desperately awaiting news of their loved ones.
Ardern said communication as a result of the eruption had been difficult but the New Zealand Defence Force and the Ministry of Foreign Affairs were working to establish what was needed and how to help.
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Ardern said the undersea cable has been impacted, probably because of power cuts, and authorities are trying urgently to restore communications.
If necessary New Zealand would help with any repairs that may be needed on the undersea cable that carries communications.
Local mobile phones in Tonga are working, however, she had not been able to reach the Tongan Prime Minister, she said.
"At the moment we are mainly receiving information from our High Commission ...unfortunately from the outer islands we don't have a lot of information," she said.
Pacific Peoples Minister Aupito William Sio said the Tongan Consul General Lenisiloti Sitafooti Aho has confirmed Tonga's Royal family are safe.
The New Zealand High Commission advises the tsunami has had a significant impact on the foreshore on the northern side of Nuku'alofa, with boats and large boulders washed ashore. Shops along the coast have been damaged and there will need to be a major cleanup, Ardern said.
While ash has stopped falling in Nuku'alofa, it is having a big impact on the island, initial reports indicate.
Authorities are still trying to make communication with some of the smaller islands, she said.
"There are parts of Tonga where we just don't know yet - we just haven't established communication."
Ardern said satellite images "really brought home the scale of that volcanic eruption," adding that people know how close Tonga is to the volcano, so it is very concerning for those trying to contact their relatives.
Sio said there had been overwhelming concern here for whānau in Tonga. Pacific people are resilient people who have experienced hurricanes and storms before and know how to respond, he said. He appealed for people to allow officials the time to ascertain how best to respond effectively.
Ardern said anyone in the Pacific region, such as holidaymakers, should heed local advice.
NZ pledges $500,000, prepares to send aid
The Tongan government has accepted a New Zealand government offer for a reconnaissance flight, and an Orion will take off tomorrow morning provided conditions allow. At present ash has been spotted at 63,000 feet.
The Tongan government has also agreed to the Australian government's offer of a surveillance flight, which will also take place tomorrow morning, pending conditions.
The government has also pledged $500,000, which is very much a starting point, Ardern said.
A naval vessel has also been put on standby to assist if necessary.
"[There is] an urgency here. We want to make sure we're on the ground as soon as possible, but for our Navy vessels it will take several days to reach Tonga, and we need to finely balance the need to get there quickly but to make sure we also get the people and resources they need there as well and in some cases, we have parts of Tonga where we just haven't been able to establish communication."
Ardern has also been in touch with Australia's Prime Minister Scott Morrison so that both governments can work in tandem in their response.
She said a priority is the supply of water for Tonga.
The reconnaisance flight will be useful to see the impact of the volcanic eruption on the low-lying islands, Ardern said.
"At the moment we stand ready to assist," Ardern said, but she added that the conditions at the moment do not make it a stable environment for aircraft to operate in.
That is why naval vessels may be needed to head to the region.
"We are preparing for those ships to sail as we speak."
The Canterbury could be deployed within eight hours, she said.
Defence Force Minister Peeni Henare said it's not known yet what has happened under the water. A New Zealand hydrographic vessel may be able to head to Tonga.
"Our people are ready to deploy. We just have to make sure they are fitted out with what the Tongan people need."
Water would be one of the critical things that New Zealand could help with, he said.
Medics, logistics staff and engineers will be the most needed job skills for staff deployed to help out, he said.