The son of two Australians injured in Monday's Whakaari/White Island eruption says the timeline for his parents' recovery is uncertain and they could face months in hospital.

Sydney couple Nick and Marion London, who are both in a critical condition, were flown back to Australia for treatment earlier in the week.

They have burns to 50 per cent of their bodies and internal chemical burns from breathing in toxic gases, Australia's Sydney Morning Herald reported.

They remain on life support and face an uncertain recovery timeline, son Matt told the newspaper.

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"The road ahead no doubt will be very difficult with many up and downs," he said.

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"Aside from a long line of surgery and recovery, all we know is that even if things go well, they'll be in hospital for weeks, if not months."

The couple's children had found it hard to reach their parents in the days after the eruption.

New Zealand Defence Force and police staff on White Island yesterday to recover bodies. Photo / NZDF
New Zealand Defence Force and police staff on White Island yesterday to recover bodies. Photo / NZDF

Initially, Matt's sister Louise was able to speak briefly to their father, who seemed in good spirits, when a nurse answered his phone and passed it on.

"To find out they were alive was of course a huge relief," Matt said. "We were so incredibly grateful to have our worst fears laid to rest."

He said he had increasingly come to appreciate how special his "gentle giant" dad and caring mum were.

"The older I get, the more I realise how absurdly lucky I am," he said.

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"Talking to other people about their families and their relationships I can see that I have been blessed to have parents who have always supported us, always had our best interests at heart, and always accepted us for every part of who we are even when that hasn't always been easy."

Matt told the Sydney Morning Herald his dad was "very strong-willed, stubborn as a mule, but always a gentle giant and will forever be our mum's knight in shining armour".

"Our mum is the very definition of kindness and care. She is one of so very few people I have ever met that have such genuine love and kindness in their hearts."

Almost $8000 has so far been raised via a GoFundMe page to support the couple's recovery.

Family friend Tom Nieuwland started the page and told the Sydney Morning Herald Nick and Marion "have an incredibly long and difficult road ahead".

Yesterday they had surgery to remove the damaged skin and had skin grafting in some areas.