Hundreds of homes in Tonga's smaller outer islands have been destroyed, and at least three people were killed after Saturday's huge eruption triggered tsunami waves, which rolled over the islands causing what the government has called an unprecedented disaster.
With its airport smothered under a layer of volcanic ash and communications badly hampered by the severing of an undersea cable, information on the scale of the devastation has mostly come from reconnaissance aircraft.
"For the people of Tonga, we're heading their way now with a whole lot of water," Simon Griffiths, captain of the HMNZS Aotearoa, said in a release.
Griffiths said his ship was carrying 250,000 litres of water, and had the capacity to produce another 70,000 litres a day, along with other supplies.
New Zealand's foreign ministry said the Tongan government has approved the arrival of Aotearoa and the HMNZS Wellington in the COVID-free nation, where concerns about a potential coronavirus outbreak are likely to complicate relief efforts.
Tonga has said its water supplies have been contaminated by ash from the Hunga Tonga-Hunga Ha'apai volcano , which erupted with a blast heard 2300km away in New Zealand. It also sent tsunami waves across the Pacific Ocean.
James Garvin, chief scientist at NASA's Goddard Space Flight Centre, said the force of the eruption was estimated to be equivalent to five to 10 megatons of TNT, an explosive force more than 500 times the nuclear bomb dropped by the United States on Hiroshima, Japan, at the end of World War II.
The Red Cross said its teams in Tonga were distributing drinking water across the islands where salt water from the tsunami and volcanic ash were "polluting the clean drinking water sources of tens of thousands of people".
A Chinese foreign ministry spokesperson said it would send help, including water and food, when the archipelago's main Fua'amotu International Airport reopens. It was not damaged but was covered in ash, which is being cleared manually,
"We thought that it would be operational yesterday, but it hasn't been fully cleared yet because more ash has been falling," Fiji-based UN co-ordinator Jonathan Veitch said on Wednesday.
Pacific neighbour Fiji will send defence engineers on Australia's HMAS Adelaide, which is due to set sail from Brisbane for Tonga on Friday, a Fiji military spokesman told a briefing in Suva.
Waves reaching up to 15 metres hit the outer Ha'apia island group, destroying all of the houses on the island of Mango, as well as the west coast of Tonga's main island, Tongatapu, the prime minister's office said.
On the west coast of Tongatapu, residents were being moved to evacuation centres as 56 houses were destroyed or seriously damaged on that coast.
New Zealand said power has now been restored, and clean-up and damage assessments were going on and Tongan authorities were distributing relief supplies.
Australia and New Zealand have promised immediate financial assistance. The US Agency for International Development approved $US100,000 in immediate assistance to support people affected by volcanic eruptions and tsunami waves.
Tonga is still largely offline after the volcano severed the sole undersea fibre-optics communication cable.
International mobile phone network provider Digicel has set up an interim system on Tongatapu using the University of South Pacific's satellite dish, the New Zealand foreign ministry said.
US cable company SubCom has advised it will take at least four weeks for Tonga's cable be repaired, it added.
