Health experts are preparing for a "nightmare scenario" in Australia, with early data from both overseas examples and the local outbreak showing the Delta strain is more severe as well as being harder to contain.
The warning comes as early data from the NSW outbreak shows more people are in hospital intensive care wards than during the peak of Victoria's second-wave outbreak, which had substantially more active cases.
The raw data from NSW is backed up by early studies out of Canada, Scotland and Singapore, which show the Delta strain appears to be associated with a higher risk of hospitalisation and death.
And last week leaked internal documents from the US Centers for Disease Control and Prevention declared "the war has changed", citing studies showing Delta caused more severe illness than earlier variants.
Greg Kelly, a Sydney-based paediatric intensive care specialist, said it was clear with the Delta strain that the things that worked once "aren't working anymore".
"Delta is testing countries and places that had really good control," Kelly said.
"Like China, Vietnam and Singapore. And in Florida, for example, it is about to pass its hospitalisation rate from their two waves last year.
"And what we are seeing is more cases in children. Local data in Australia shows about 25 percent of new cases are in children, and we are treating more children at our hospitals.
"In the US they are seeing substantial numbers of critically-ill children, and that could be a sign of things to come."
As of Monday, the NSW government had cancelled non-urgent elective surgery at public hospitals across Greater Sydney. And the ABC understands hospitals across Sydney are redeploying some medical staff in preparation for an influx of patients if cases continue to rise.
Kelly said, although he believed hospitals were "more prepared" than last year, they were still bracing themselves for a "nightmare scenario".
"The reality is [in NSW] the cases are still rising," he said.
"We're planning for large numbers of critically unwell patients with Covid.
"We have a three-stage plan and the final stage would be converting the entire ICU to a Covid ward. I really hope we don't see that in Australia.
"We do have the benefit of being able to look overseas and we do have the benefit of public health and government leadership at all levels. But we are making those preparations."
NSW and Victoria
On August 19 last year, the Victorian government reported 675 people in hospital - the peak of hospitalisations during Victoria's second wave.
Of those, 44 were in ICU and 29 of those were on ventilation. Victoria's second wave, which killed 768 people, reached a peak of 6768 active cases on 7 August last year and was the original Wuhan strain of the virus.
According to NSW Health data on Monday, 232 Covid-19-positive patients were in hospital, 54 of whom were in ICU and 25 of those on ventilation. All of NSW's 3038 active cases are believed to be the Delta strain.
However, according to clinical epidemiologist Nancy Baxter it is "very difficult" to compare the two outbreaks and the hospitalisation data.
"Unfortunately, a lot of the deaths in Victoria's second wave were in the elderly and aged care, and those weren't people that were ever going to go into ICU," Professor Baxter, from the University of Melbourne, said.
"Now, [in Sydney] you have a larger group of people getting infected who can actually benefit from ICU, so you'll see more people present. And right now the older folk are more protected with vaccination, the younger people don't have that."
Delta's 'double whammy'
However, Professor Baxter said despite her hesitation to compare the two outbreaks, she believed it was "very likely" the Delta strain was more severe.
She said the reports from Canada, Singapore and Scotland demonstrated that the Delta variant seemed to be associated with a higher risk of hospitalisation, a higher risk of ICU admission and a higher risk of death.
"And those risks seem to be at least twice as high as previous variants, in some cases, even higher than that," she said.
"So there does seem to be a strong signal at this point that it is more serious if you have it. [And] if more people get it, more people are going to be seriously ill.
"So this is a double whammy that makes it more concerning."
Professor Tony Cunningham, from Sydney's Westmead Institute for Medical Research, said it was becoming "pretty clear" from overseas studies that Delta appeared to lead to a "two-to-three-fold increase in hospitalisation".
"Delta is different," Professor Cunningham said. "The risk equation has changed. There's growing evidence that it actually causes disease more readily and that it's spread more quickly after initial contact.
"And that means there's more chance for invisible spreading in the community."
Professor Cunningham, an infectious diseases physician, clinical virologist and scientist, said despite some queries over the longevity of vaccines, they were still the one clear element needed for protection.
"We know Delta's far more contagious, and we know it's present in more than 120 countries," he said.
"It has taken over in the UK, India, the US and Indonesia.
"So get yourself immunised, fully immunised, and abide by the health regulations."