There are 536 new cases of COVID-19 in New South Wales - including nine of the Omicron variant - amid a spiralling outbreak linked to a Newcastle nightclub, as new research finds three people in Melbourne contracted the virus for a second time.
With the festive season well underway, NSW Health is battling to contain several large transmission events, including some with Omicron cases.
There are now 64 cases of the new variant recorded in the state.
"There were nine new cases confirmed with the Omicron variant overnight, and more are expected as results of genomic testing are confirmed, NSW Health said.
The number of cases linked to an outbreak at Newcastle's Argyle House nightclub spiked from 24 to 84 overnight and "a number of these cases are likely to be the Omicron variant".
Four people who partied at the club on Wednesday had COVID-19, with the cases linked to the outbreak stemming from a Sydney Harbour Cruise of 140 people on 3 December.
All 680 people who checked into the Newcastle nightclub with a QR code between 9pm on Wednesday and 3am on Thursday are close contacts and must immediately get tested and isolate for seven days.
Some 74,997 COVID tests were recorded in NSW in the 24 hours to 8pm on Sunday and there were no further deaths.
The double-dose vaccination rate of those 16 and older remains at 93.1 per cent, while 94.8 per cent of people have had their first jab.
There are 171 people are in NSW hospitals with the virus, 24 of them in intensive care.
Some 81.4 per cent of teens aged 12-15 have had one dose of a COVID-19 vaccine, while 77.7 per cent are fully jabbed.
It comes as Victoria reports 1,290 new COVID-19 infections and a further two deaths.
The state is now managing 11,649 active cases despite 92 per cent of residents aged 12 having got two doses of a COVID-19 vaccine.
There are 323 patients in hospital, 77 of whom are actively infected with the virus in intensive care and 40 needing ventilation. The seven-day hospitalisation average sits at 312.
Testers processed 55,122 results on Sunday, while 3,266 people were vaccinated in state-run hubs.
The latest figures come as medical researchers discovered three people from a Melbourne household had contracted COVID-19 for a second time, one year after first becoming infected.
A health worker at a COVID-19 testing site at Ascot Vale in Melbourne Source: AAP
They tested positive for the virus in July 2020, with two of three suffering mild symptoms and the third reporting no symptoms, according to a study published in the Medical Journal of Australia on Monday.
The variant they contracted, pangolin lineage D.2, was not of concern.
Then all three contracted the Deltra strain of COVID-19 in July this year, but only experienced mild symptoms.
While second infections are believed to impact less than one per cent of people, the researchers are urging people who have contracted the virus to still get vaccinated.
"Just because you've had COVID-19 and recovered from it doesn't make you immune. You're still at risk of getting COVID-19. Our only protection at the moment is the vaccination," Monash University epidemiologist Mohana Baptista said.
Victoria has now also lowered quarantine requirements for travellers from several countries of concern.
Fully vaccinated travellers from South Africa, Botswana, Eswatini, Lesotho, Malawi, Mozambique, Namibia and Zimbabwe are no longer required to undertake 14 days in hotel quarantine upon arrival.
They must instead self-quarantine for 72 hours, get tested within 24 hours of arrival and again between days five and seven, like other international travellers.
The state is awaiting the test results of several passengers from an international flight that carried at least two confirmed Omicron cases.
There are three Omicron cases in Victoria - a returned traveller from the Netherlands who is in hotel quarantine and two people who sat together in the same row on a flight from Dubai to Melbourne on November 30.
A third person who also sat in the row has tested positive for COVID-19 and genomic sequencing is underway to confirm the variant.
Four other passengers on the same flight and three household contacts of a case are also being tested.
The ACT, meanwhile, has recorded three new cases of COVID-19, bringing the total number of active cases in the territory to 63.
There are four people in hospital, two of whom are in intensive care but neither require ventilation.
More than 98 per cent of ACT residents aged 12 and over are fully vaccinated.
South Australia has also recorded 13 new cases of COVID-19.
"I think our state should now be ready to have these types of numbers pretty much on a daily basis as we go into this next phase on the disease," Premier Steven Marshall said at a press conference on Monday afternoon.
"My understanding is the vast majority of them are Delta," the premier added.
