On Tuesday, New Zealand's acting High Commissioner Peter Lund said a state of emergency had been declared in Tonga.
A massive clean-up operation is under way on the main island Tongatapu and the capital Nuku'alofa, blanketed in ash from the blast.
Mr Lund confirmed reports the body of missing 50-year-old British woman Angela Glover had been found, and the likelihood of two other deaths.
"I understand in talking to our police advisor this morning, unconfirmed reports of up to three fatalities," he told NZ television station Tagata Pasifika.
"Here on Tongatapu, it is very sad some fatalities have been reported.
"We were spared great loss of life. Most people are safe and now having to face a big clean-up and taking things day by day."
As of Tuesday afternoon, NZs Ministry of Foreign Affairs and Trade confirmed two deaths, including a British national understood to be Ms Glover.
On Monday, New Zealand's military sent a P-3K2 Orion aircraft into Tongan airspace on a reconnaissance mission, feeding back its findings to the Tongan government.
"Imagery and details have been sent to relevant authorities in Tonga, to aid in decisions about what support is most needed," NZ foreign minister Nanaia Mahuta said.
However, the country's main airport remains off-limits to incoming flights due to ashfall on the runway.
In the meantime, NZ is sending both the HMNZS Wellington and HMNZS Aotearoa on the three-day sail to Tonga beginning on Tuesday.
Defence Minister Peeni Henare said the Wellington would carry hydrographic survey and diving teams and a Seasprite helicopter, and the Aotearoa would take "bulk water supplies and humanitarian and disaster relief stores".
"Water is among the highest priorities for Tonga at this stage and HMNZS Aotearoa can carry 250,000 litres, and produce 70,000 litres per day through a desalination plant," he said.
A Hercules C-130 aircraft will also deliver relief once the runway is cleared.
Mr Lund said the "huge clean-up operation" from widespread ashfall had begun in earnest.
"The government here has declared a state of emergency,' he said.
"They are used to dealing with tropical cyclones. A tsunami ... is a whole new challenge. They're making progress. Roads are being cleared. Buildings are being cleaned up ... Nuku'alofa is trying to get back to normal.
"There's a priority on getting the runway cleared so that emergency supply flights ... from NZ and Australia can enter the country."
The New Zealand government has also upped its initial aid allocation to $NZ1 million ($A940,000), promising further assistance to come.
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