NSW and Victoria recorded 3803 new infections between the country's two most populous states on Monday.
Case tallies are expected to rise further as people take advantage of open borders across much of the country over Christmas.
University of New South Wales professor of epidemiology John Kaldor cautions a lot remains unknown about the Omicron strain.
"The first thing to figure out, as with any new variant, is first of all how infectious (is it)? Then, secondly, how much disease is it causing and, particularly, how well are the vaccines working?" he told ABC radio.
"We've got the answer to the first one of those three, but we haven't got the answer to the second and the third as yet."
Professor Kaldor warned waiting to see whether hospitalisation rates worsened before beefing up public health measures was was risky.
"We can't be waiting to watch that number (and) react in some way to make our decisions at this stage," he said.Â
He expected new infections to climb into the tens of thousands in the absence of additional infection controls, particularly in NSW.
"If the severity is of a scale that's even half that of Delta, then there'll be a really strong impact on the hospitals and their ability just to provide care to people who need it," Prof Kaldor said.
There were 2501 new cases in NSW reported on Monday, with 261 people in hospital - 33 of them in intensive care.
Victoria recorded 1302 additional infections. The state has 406 people in hospital including 81 in intensive care.
Deputy Prime Minister Barnaby Joyce, who contracted Omicron during an overseas trip to the United Kingdom and United States, said people needed to get on with their lives.
"It was really mild, a couple of days and the rest of the time you had the virus, you didn't feel sick. And that is one of the frustrations," he told the Seven Network.
Mr Joyce, who is double vaccinated, has been cleared to leave hotel quarantine in Washington and return to Australia.
Epidemiologist Adrian Esterman called on the Australian Technical Advisory Group on Immunisation to recommend boosters three months after a second dose.
He feared leaving the gap at five months will result in "thousands of people infected for no reason".
The University of South Australia professor anticipated booster shots becoming an annual event, if not more frequent than that.
South Australia on Sunday reported 80 new COVID-19 cases, Queensland 42 and the ACT 18.
There were nine new infections in the Northern Territory as authorities there anticipated extending a lockdown in the town of Tennant Creek.
Tasmania reported three new cases as it prepared to mandate masks indoors.
Australia's full vaccination rate for people aged 16 and older is sitting at 90.42 per cent. The first dose rate is nearing 94 per cent.
The number of boosters delivered into arms exceeds 1.3 million, with more than 640,000 doled out in the last week.
11°C