New South Wales Premier Gladys Berejiklian has warned of tough new coronavirus restrictions for Greater Sydney as the city's cases increased by 10 overnight.

The outbreak — which is predominantly spreading in Sydney's Northern Beaches, prompting Berejiklian to issue a localised 'stay at home' warning — has now grown to 28 cases.

The advice, which came into effect for the Northern Beaches area of 250,000 people yesterday, is asking residents to stay in their homes for all but essential reasons. "Do not leave your home", Berejiklian implored.

The Premier had earlier warned the state to "brace themselves" and expect more cases as contact tracers race to get on top of the cluster.

Berejiklian said there was potential seeding out of the region after people who visited the Northern Beaches returned to their homes around Sydney.

"The number of people who were at the (Avalon) RSL event on December 11 and then the (Avalon) bowling club on Sunday were substantial and obviously they have got household contacts and contacts of theirs," she said.

The Premier said she didn't want to "go down the mandatory path" and order additional restrictions – but she would if necessary.

"I will if we have to," she said.

"I'm hoping it won't get to further measures, let's just see the next 24 to 48 hours, but we won't hesitate to take on health advice.

"We believe what we have in place matches the risk."

NSW Health Minister Brad Hazzard had some harsher words for the state, describing the outbreak as the result of "an avalanche of complacency".

"We're in the middle of a worldwide pandemic … the Avalon cluster has served as very timely reminder that there is no room for complacency."

Queensland

Queensland Premier Annastacia Palaszcuk today announced three new cases for the state.

Two are in hotel quarantine and the final case is the woman linked to the Northern Beaches cluster, who travelled from Sydney to Queensland on Wednesday.

A number of health alerts have been issued for Queensland after the woman tested positive last night.

The woman is now self-isolating back in Sydney.

Queensland will enforce mandatory hotel quarantine from Saturday for anyone who has been in the Northern Beaches region, while those who have already entered have been told to isolate for 14 days from their last visit to the hotspot.

Queensland chief health officer Dr Jeannette Young said her team will continue to watch as the outbreak develops.

"We just have to continue to be really cautious," she said.

"This is a rapidly evolving situation in the Northern Beaches and could well expand through other parts of Sydney."

"Superspreader event"

One expert warns the explosion of new cases is likely linked to a single superspreader event.

Professor Nancy Baxter, a clinical epidemiologist from the University of Melbourne, said the outbreak was "extremely concerning" and warned cases will likely continue to rise.

"It looks like the super spreading event likely happened December 11," she told ABC's News Breakfast.

"You've now had seven days where this has been circulating. Although a number of people have been linked to the cluster, we don't know who the original source was.

"That person has been circulating for that period of time. I would anticipate a number of additional people will be identified as having Covid-19."

Baxter said the person likely had mild or no symptoms and might no longer be infectious but was probably unknowingly spreading the virus for a significant period of time.

NSW chief health officer Dr Kerry Chant today said the spread of infections "clearly shows we've had a significant seeding event occur, we think, on December 11".

Berejiklian revealed this morning genomic sequencing indicated the virus was from an overseas source and said the NSW community would get any information about affected venues or people "as soon as I do".

International aircrew arriving in Sydney

From Tuesday, international aircrew landing in Sydney will be put into police-managed hotel quarantine.

Berejiklian said the previous arrangement allowed aircrew to stay at more than 25 hotels across Sydney.

Now, however, because of a handful of people "disobeying what's in place", aircrew would be taken to two hotels near the airport, staffed by NSW Health and police.

Berejiklian appreciated it would "put pressure on aircrew and airlines" but the state needed to react to recent virus leaks.

"The issue isn't the guidelines, it's unfortunately when people breach the guidelines or choose not to self-isolate," she said.

"It's people disobeying what's in place, so there'll be no chance of disobedience."