Prime Minister Jacinda Ardern said it's "too early to tell" whether anyone has lost their life in the volcanic eruption and tsunami that hit Tonga, although so far there were no official reports of deaths of injuries.
A defence force plane is ready to fly out, and may leave tomorrow, and a New Zealand Navy vessel is on standby to provide assistance if required.
Ardern was speaking in Auckland with Defence Minister Peeni Henare and Pacific Peoples Minister Aupito William Sio, who laid out the Government's first response to the crisis.
She said one of the first requests from Tonga was for water, as the ash cloud from the eruption had contaminated water supplies.
Ardern said "at this stage it would be too early" to tell whether there were fatalities.
"I know many of you will have seen the footage and found it deeply disturbing," she said
"Being able to see those satellite images brought home the scale and the violence of that eruption," Ardern said.
"I have family who have ties and links and who work across the region and have satellite communication so that's where I heard some of that emerging communication," Ardern said.
She said New Zealanders working for the Government in Tonga were safe.
Sio said "we've got MPs who have linkages back to Tonga, and they are worried".
"The Pacific are part of our family and so there's been overwhelming concern from the diaspora here," he said.
Sio said the impacts of the volcano had been felt across the region.
He said the Consul-General in Tonga had reported that the royal family was "well and fine".
The High Commission said north Nuku'alofa had been impacted, with large boulders washed ashore.
Ardern urged New Zealanders in Tonga to keep family informed of their wellbeing. Local mobile phones are working, she said.
"Please keep family back in New Zealand informed of your wellbeing if you can".
While the main undersea cable had been impacted by the disaster, local mobile phones were working.
"The main undersea communications cable has been impacted, likely due to the loss of power, and that is why communications have been limited," Ardern said.
The Government cannot rule out further volcanic activity.
"I've been in touch with Australian Prime Minister Scott Morrison today and New Zealand and Australia stand ready to assist our Pacific neighbours," Ardern said.
"We have the aircraft ready to deploy and are looking at the deployment of a Navy vessel should it be required," she said.
Henare said the defence force was ready to deploy.
"We are talking now about what's happening above water, we don't know what is happening below water," he said.
"Our people are ready to deploy," Henare said.
The C130 Hercules aircraft is also ready to deploy to deliver goods, but this would require making sure it was safe to land in Nuku'alofa, and waiting for ash to disperse to be safe for flying.
Ardern said flights might depart tomorrow "we hope".
"We will make sure that as soon as we know that flights are able to be undertaken we will be staying in touch with, in particular, Australia and Fiji," Ardern said.
Ardern said the Navy's Canterbury vessel, which carried desalination equipment to provide fresh water, could deploy within eight hours.
An initial $500,000 of aid has been committed from New Zealand as "a starting figure".
"The Government will provide additional assistance as required," Ardern said.
National leader Chris Luxon said the opposition "shares concerns for the people of Tonga and supports New Zealand standing ready to provide assistance".
"Our thoughts are with the people of Tonga as they deal with the damage and distress brought about by the recent tsunami that followed a volcanic eruption," he said.
"I know that there will be many people around New Zealand tonight waiting to hear about their family, friends and loved ones while communication remains unstable, and our thoughts are with them too.
Luxon said National's Pacific Peoples spokesperson Shane Reti "has reached out to local Pacific leaders, the Tongan Consul, and Pacific Peoples Minister Aupito William Sio on behalf of the Opposition to express our sympathy and offer our support".
He suggested that it might be appropriate to dispatch "the multi-purpose Aotearoa vessel if planes are not able to safely fly over the volcanic ash cloud".
In an Instagram post, Ardern said images of the volcanic eruption are "hugely concerning".
"Communication as a result of the eruption has been difficult, but our defence force team and Ministry of Foreign Affairs [Mfat] are working as we speak to establish what's needed and how we can help," Ardern said in the post.
Officials have already mobilised the first phase of the Government's response.
A spokeswoman from Mfat said "damage assessments are currently under way, and New Zealand has formally agreed to provide assistance".
"An NZDF P3 Orion is on standby to provide aerial surveillance as soon as atmospheric conditions allow," they said.
"The New Zealand High Commission in Nuku'alofa is monitoring the situation closely and is in contact with local authorities."
The spokeswoman said there were currently no official reports of injuries or deaths, but cautioned that current communications were limited.
Currently, 30 New Zealanders are registered on SafeTravel as being in Tonga. Mfat advised New Zealanders in Tonga to "follow the advice of the local authorities, including any tsunami evacuation orders".
Mfat said New Zealanders requiring consular assistance should contact the New Zealand Consular Assistance line on +64 99 20 20 20.
They warned that communications links with Tonga have been disrupted so New Zealanders may have difficulties contacting their whānau in Tonga at this time.