Specialist New Zealand Defence Force staff will be checking Tonga's shipping lanes are passable and the wharf is safe so desperately needed humanitarian supplies can get through.
Three deaths have been confirmed after Saturday's massive volcanic eruption. There are reports of significant injuries, but no further details yet.
UN officials said 84,000 people - more than 80 per cent of Tonga's population - had been impacted by tsunami and the ashfall that followed the eruption.
New Zealand Defence Force Commander of Joint Forces Rear Admiral Jim Gilmour said there were fears for food security, with reports ash was killing crops.
Ash and seawater have also contaminated water supplies.
Offshore patrol vessel HMNZS Wellington, which is carrying a helicopter, technical gear, and teams, has arrived in Tongan waters.
It will scope the shipping channels and wharves at the main port to see if they are safe enough to use to drop off supplies, in time for HMNZS Aotearoa, which is carrying bulk water supplies, and is due today.
"Water is among the highest priorities for Tonga, and the Aotearoa can carry 250,000 litres, and produce 70,000 litres per day through a desalination plant."
Gilmour said staff did not need to set foot on Tonga at all, in an effort to avoid spreading Covid-19 to the currently coronavirus-free country.
Sanitised containers will be moved by crane from the ship on to the dock or hauled by personnel in full PPE.
They will then withdraw and Tongans will pick up the goods.
Hundreds of people, including the Tongan Armed Forces, cleared ash off the international runway allowing an RNZAF Hercules to land yesterday afternoon.
It carried the most urgently needed supplies including water containers, temporary shelters, generators, and communications equipment.
It was expected to be on the ground for about 90 minutes before returning to New Zealand.
It could head back to Tonga as soon as Saturday and could make at least three return flights.
A Royal Australian Air Force C-17 also landed on Thursday.
A third New Zealand Defence Force vessel, HMNZS Canterbury, is being prepared to be deployed on Saturday to arrive on Tuesday.
It is carrying two helicopters which can be used to distribute supplies and survey Tonga's outer islands.
The Defence Force intends to be self sufficient to not put pressure on Tonga's food, water and fuel supply.
It has enough stores to stay at sea for at least 30 days without any external assistance. If it stays that long plans will be made to resupply.
Australia's high commissioner to Tonga Rachael Moore has described the loss of property as "catastrophic".
"Along the western beaches there is a moonscape where once beautiful resorts and many, many homes stood," Moore said.
Tonga has only just begun to re-establish global contact after five days cut off from the rest of the world.
Mobile phone company Digicel has confirmed re-establishing communications between Tonga and the rest of the world, but lines have been clogged with heavy traffic, leaving many still unable to get through to loved ones.
Work to improve the satellite capacity and improve communications at the New Zealand High Commission in Nuku'alofa was being done Thursday evening.
• Contribute to the Red Cross appeal for Tonga, here.