Rain mixing with the heavy volcanic ash lying on the island could encase the bodies in a cement-like substance, making recovery more hard.
A coronial process has begun to identify the eight confirmed dead but police said it could "take some time".
Matthew and Berend Hollander, aged 13 and 16, died in hospital on Wednesday, the headmaster of the Sydney grammar school they attended announced, calling it "a devastating loss for our community".
The official death toll is eight, and eight other people are on the island and presumed dead.
Graham Leonard, a senior scientist at New Zealand's geological science agency GNS Science, said monitoring equipment still active on the island put the risk of a further eruption over the next 24 hours at 50% to 60%, up from 40% to 60% on Wednesday.
Their deaths mean 28 people are now being treated in hospital - 21 in New Zealand and seven in Australia - some with burns on up to 80 per cent of their bodies. Its alert level since late Monday had been 3 on a scale where 5 signifies a major eruption.
New Zealand police will undertake an operation to retrieve bodies left behind following this week's volcanic eruption on White Island that killed at least eight people and injured dozens more.
'The risk is not gone. Some have told RNZ they are desperate for the bodies of those missing on Whakaari, which is 50km off shore, to be recovered today.
Defence force personnel landed on the island on Friday morning, along with assistance from police and other agencies in the recovery mission that could take several hours, police said.
Royal Caribbean cruise lines has said that passengers from its ship "Ovation of the Seas" were on the island at the time.
Six bodies could be seen and there would be "very limited" opportunity to search for the other two, he said.
The risks were still high - the volcano on White Island was "highly volatile" and could erupt again within days, experts have warned. Twelve of the injured Australians were being medically evacuated to Australian hospitals for further treatment with one Australian patient staying in a New Zealand hospital, he said at a news conference Thursday. He said he is concerned about the weather, the direction of the wind, and the state of the sea, as they all pose a danger.
"A lot has to go right for us tomorrow to make this work", said Deputy Commissioner Mike Clement.
Eight people have been officially declared dead, while more than 20 are being treated for severe burns.
The volcano spewed hot ash and toxic gas Monday as 47 tourists from around the globe were visiting White Island.