Kiwi officials made the agonising call in a meeting of the assembled Science and Technical Advisory Committee on Wednesday.

National Emergency Management Agency director Sarah Stuart Black said she understood the desire of loved ones to see the return of bodies but conditions were simply too unstable.

"This tragedy has already claimed multiple lives," she said.

"There is a delicate balancing act where the threat to human life exists and right now the science tells us that is too high."

Senior volcanologist Graham Leonard said GNS monitoring of White Island showed "further escalation" in tremors overnight, which has continued through Wednesday.

"Yesterday there was a there was a high risk of eruption, today there was an even higher risk of eruption and the parameters are worsening," he said.

"Seismic signals on the island are escalating at the moment .. there's a risk of 40 to 60 per cent of another eruption, like Monday's, in the next 24 hours."

Leonard said the environmental on White Island would be "challenging for breathing, seeing and walking" already.

The alert level remains at three, signalling "minor volcanic eruptions".

"It's important to remember Whakaari/White Island is New Zealand's most active volcano and there remains significant uncertainty about any future activity," volcanologist Craig Miller said.

The situation is being reassessed every few hours, though the GNS prognosis suggests it won't provide an opportunity for a retrieval effort until late Thursday at the earliest.

Others suggest a less cautious approach.

A helicopter pilot who flew to the island on Monday after the eruption as part of rescue efforts, Mark Law, gave a television interview saying "conditions were perfect for recovery".

"For us, it's 20 minutes to get out there. We could load those folks on and be back here in an hour and a half," he told Channel Three.

"I know where they all are, and the conditions are perfect for recovery in my mind."

Police have defended the decision to defer the operation, and maintained their commitment to retrieving the bodies.

"We have disaster victim identification specialists in Whakatane ready to be deployed," National Operation Commander John Tims said.

"It is a number one priority. We will return to the island, we will recover the people there."

Earlier, prime minister Jacinda Ardern said it was "everyone's hope" that this could take place on Wednesday.

"I've spoken to many of those involved in the operation and they are very, very eager to get back there.

"They want to bring people's loved ones home."

Australian Associated Press