Australia's coronavirus record is unravelling with 16 new cases today, 10 in NSW and 6 in Victoria.

Ten new virus cases have been revealed in New South Wales, with five linked to the northern beaches cluster.

Three are associated with the growing new outbreak in Croydon after several members of the same family tested positive yesterday, sparking fears the virus is spreading across Greater Sydney.

"One is a link to the transport worker cluster which we previously have spoken about and one is under investigation," NSW Premier Gladys Berejiklian said.

"There's a chance it's an old case or else a false positive but serological is looking at that."

Meanwhile, Victoria has recorded six new cases, with the state returning to harsh restrictions within hours in a bid to stop the spread.

Authorities across both states have pleaded with the public to avoid New Year's celebrations this evening amid fears it could lead to "superspreader" events, and have urged the public to take the current threat seriously and to follow public health advice.

In response to the outbreak, Sydneysiders, and those on the Central Coast, Blue Mountains and Wollongong, are only allowed to have five visitors over for the New Year's Eve celebrations. Outdoor gatherings will be limited to 30 people.

Health minister Brad Hazzard slammed comments made by the WA Australia Medical Association president Dr Andrew Miller who said NSW was no longer the gold standard and a lockdown "wouldn't be going too far".

"If the (WA) AMA president has a particular view, I will leave that to him in WA. He must be an instant expert on what is happening here in NSW," he said.

"I'm happy to just continue to work with our health authorities who have done a fantastic job."

There were 18 cases yesterday, with nine of those linked to the northern beaches outbreak.

The Croydon cluster

NSW Health is still unclear over the origins of the Croydon cluster, however Dr Kerry Chant said she's expecting a result later this afternoon.

Sharing more details about the Croydon cluster, Dr Chant said members in the three households linked to the original cluster have attended a "number of functions" that occured between Christmas and Boxing Day.

The cases were also in the community while unknowingly infectious, although it's still "too early" to tell if there's been transmission as a result of this.

A man receives a Covid-19 test at a drive-thru testing station at a beach in Sydney. Photo / AP

"So it means, and I'm very sorry for this, but your New Year's Eve plans will have to change," she said.

Dr Chant continued to emphasise the importance of testing and implored NSW residents to not attend gatherings of any kind if symptomatic.

"Please can I just put out a plea that do not attend any family events, family gatherings, New Year's Eve gatherings, go out and about, if you get symptoms," she said.

"I think we want to all see a much brighter 2021 and we do have some hope on the horizon in terms of vaccines.

"We all have a part to play. So thank you for following that advice, please."

Victoria cases rise

Three additional Covid-19 cases have been uncovered in the community in Victoria taking the total number to six, Acting Premier Jacinta Allen said today.

She said the three additional cases were uncovered through contact tracing after three positive cases were discovered in Victoria yesterday.

The first three cases were linked by a Thai restaurant in Black Rock that was also linked to a NSW returned traveller, Health Minister Martin Foley said.

"In less than 24 hours, the crossover between the Mitcham and the Hallam and the Mentone people has been a facility in Black Rock, and a direct link to a NSW returned traveller," he said.

"Now that we've got links to the NSW outbreak here in Victoria, we're responding really, really quickly to get on top of that."

The three additional cases today were linked to that cluster and were uncovered through contact tracing, he said.

Anyone who visited that Smile Buffalo Thai restaurant on December 21 is urged to get tested immediately.

The first three cases come from the Melbourne suburbs of Mitcham, Hallam and Mentone — all of whom visited the same venue in Black Rock, it has been revealed.

Victoria's Commander of Covid-19 response Jeroen Weimar said the state was "deploying our full outbreak approach".

"We have been in this position before," he said.

"Extensive contact tracing is underway and as a result there are currently more than 40 primary close contacts that are being supported to isolate immediately."

He said the three positive cases are self-isolating at home.

They are two women in their 40s and a woman in her 70s.

Five locations have been designated high risk, after they were visited by the Covid-positive people while they were potentially infectious:

• Black Rock: Smile Buffalo Thai restaurant on December 21
• Doveton: Holy Family Parish Doveton Catholic Church on December 26 between 4pm and 6pm
• Fountain Gate Shopping Centre: Kmart, Big W, Target, Millers, King of Gifts, Lacoste on December 26 between 9am and 11am
• Mentone: Mentone/Parkdale Beach on December 27 between 10am and 4.30pm
• Glen Waverley: Century City Walk and Mocha Jo's on December 28 between 1.30pm and 5pm
• Oakleigh: Katialo restaurant, Eaton Mall on December 28 between 7pm and 8.15pm

Victoria also introduced new border restrictions yesterday.

Anyone who has been in or visited the Blue Mountains or Wollongong regions from 27 December has until 11.59 on New Year's Eve to get to Victoria.

Nobody who has visited these areas will be able to enter Victoria after December 31.

A couple walk along the waterline on Christmas Day at Sydney's Bondi Beach. Photo / AP

Anyone intending to return to Victoria from these areas before midnight must apply for a new travel permit through Service Victoria, must get tested within 24 hours of returning to Victoria, and must self-quarantine at home for 14 days from when they last left the region.

It follows one woman testing positive last week after returning from a trip to the northern beaches, Sydney's Covid-19 epicentre.

Other positive Covid-19 cases in Victoria recorded in the past two months were from returned travellers in hotel quarantine.

Restrictions in Victoria

Masks will be mandatory indoors in Victoria from 5pm today outside of your own private residence, Acting Premier Jacinta Allan announced.

"If you are planning to leave your home at the moment, we ask people to carry their masks with them, we are now asking them to wear a mask if they are indoors in any location which is not their private home," she said.

The limit of people gathering in private homes will also be reduced, from 30 down to 15.

The limit on indoor gatherings will also apply from 5pm on New Year's Eve, Allan said.

"If you are planning a party tonight, you will need to reduce your numbers from the 30 down to 15," she said.

"We do apologise to people who have put plans in place, who was looking forward to having events in their home, or in other people's homes, but this is a necessary step.

"Unfortunately, we know with this virus, is rampant difficult virus, it does not end at midnight tonight."

No changes to outdoor gatherings, hospitality venues and shops have been announced.

Melbourne residents have previously been advised not to travel into the city for new Year's Eve unless they have a booking.