There is significant damage to the western coast of Tonga's main island of Tongatapu.
The New Zealand High Commission says the damage is centred along the western coast, where there are many resorts, and the waterfront of the capital, Nuku'alofa.
A thick layer of ash remains blanketed over the main island, the High Commission said.
"The whole western coastline has been completely destroyed along with Kanukupolu village," the owners of Ha'atafu Beach Resort said on Facebook earlier today, reporting that "our beautiful home Ha'atafu Beach Resort has been completely wiped out".
Authorities are working to establish communication with smaller islands "as a matter of priority", the commission said.
The commission advised any New Zealanders in Tonga to register with www.safetravel.govt.nz, and urged them to follow advice from local authorities, including any tsunami evacuation orders.
The Defence Force carried out a surveillance flight of the islands today. Plans are underway for a humanitarian relief flight when conditions allow.
The Royal NZ Air Force Orion which left for Tonga to assess the damage caused by the mammoth volcanic eruption over the weekend returned jut before 6pm.
The aircraft left for Tonga early this morning and is due to send back images and video footage of the damage.
Crew members were sent to assess the situation and determine exactly what help is needed, initially.
The situation has been hampered by the fact that communications networks in the island nation have been cut for close to 40 hours since the eruption and Tongans from around the world have struggled to make contact with loved ones back home.
The New Zealand Defence Force has an RNZAF C-130H(NZ) Hercules on standby to fly to Tonga tomorrow to deliver aid supplies if is safe to land at the runway on Tongatapu.
"We have placed personnel on shortened notices to move and Royal New Zealand Navy ships are being readied for deployment and may deploy ahead of a formal request for assistance, given the distance to Tonga," a spokesperson said.
As New Zealand prepares to help Tonga in its recovery - Prime Minister Jacinda Ardern has already committed an initial $500,000 - a New Zealand Navy vessel is on standby to provide assistance if required.
Earlier today, Ardern, joined by Minister of Defence Peeni Henare, told media that the arrival of a Royal NZ Air Force Orion in Tonga is now imminent.
She also acknowledged that supplies are ready to go - no matter what the status of the main airport in Tonga is.
On the Tonga Hunga Tonga-Hunga Haʻapai eruption, Ardern said an Air Force Lockheed C-130 would be dispatched. Another aircraft, a P-3K2 Orion, was already monitoring the situation.
Henare said it was important to ensure any vessels or craft deployed would need to have contact with authorities in Tonga.
Ardern said they have been communicating with people using satellite phones and information from people on the ground through other avenues - including via church ministers and familial links.
"We are getting back critical information that's helping with the planning."
The stand-up comes as the Government waits to get word on the extent of the damage caused by a giant volcano eruption late on Saturday afternoon, which resulted in tsunami waves wiping out homes on the coast and sending locals to higher ground.