More than 3800 new daily cases have been recorded across NSW and Victoria combined, fuelled by open borders and Christmas travel.
It comes as deputy Chief Medical Officer Sonya Bennett warns Omicron transmission has a doubling time of around two days.
She has urged people to don masks while indoors, stick to outdoor Christmas gatherings and limit numbers.
"The transmission rate alone is concerning. And if we see high numbers, that sheer number of cases is a cause for concern," Dr Bennett told reporters on Sunday.
"If we do end up with a lot of cases, regardless of severity, that's going to clearly have an impact on both business and industry, and individuals."
But Federal Health Minister Greg Hunt did not anticipate a return to COVID-19 lockdowns.
"We're going into summer and we have one of the highest vaccination rates in the world and a very different set of circumstances," he said.
Other countries are ramping up COVID-19 restrictions and the Netherlands is reimposing a lockdown.
"We don't see that that's a likely situation in Australia," Mr Hunt said.
The minister urged people to get their booster shot as soon as they were eligible, five months after a second jab.
About 1.3 million people have received their booster shots, with more than 640,000 doled out in the last week.
Australia has also surpassed a 90 per cent full vaccination rate for people aged 16 and older.
Infectious diseases pediatrician Robert Booy did not expect infections to fall until February following an anticipated January peak.
"There are so many simple things that are effective that could be reinstituted that won't affect people's lives too much," he told the ABC.
Professor Booy wants people to wear masks indoors and for contact tracing to be scaled up, not down.
He also urged NSW to bring back QR code check-ins en masse instead of reserving them for settings classified as high risk.
"There's all sorts of things like that that can be done which can make a difference without having to fully lock down," he said.
NSW recorded 2566 new infections on Sunday. There are 227 people in hospital, 28 of them in intensive care.
Victoria reported 1240 new cases and four more deaths. It has 392 COVID patients in hospital, 81 of them in intensive care.
The ACT recorded 18 new cases. There are three people in hospital but none in intensive care.
Even if Omicron infections proved to be less severe, Prof Booy warned higher transmission rates meant just as many people could end up in hospital.
On Saturday, South Australia recorded 73 new infections and Queensland 24 locally acquired cases.
There were eight new cases in the Northern Territory and one in Tasmania.
23°C