NSW Health authorities have confirmed  the COVID-19 cluster in Sydney’s Northern Beaches has blown out to 17, with the source of the infections still a mystery.

Fifteen new cases were identified on Thursday, with health authorities urging locals to stay at home as much as possible over the next three days.

NSW Health says residents should not visit hospitals and aged care facilities unless essential, avoid unnecessary gatherings, keep to their household groups, and avoid high-risk venues like gyms.

People should avoid unnecessary travel to the Northern Beaches and people living there should avoid leaving.

The new cases are still being interviewed and more cases will probably be identified, NSW Health warns.

Authorities are investigating a potential super-spreader event at a local RSL, Chief Health Officer Kerry Chant says.

Health authorities detailed three new cases on Thursday morning – a woman in her 50s who works at Pittwater Palms aged care facility at Avalon Beach, her partner and a Frenchs Forest man in his 60s who plays the drums for band Nothing-Too Serious.

Dr Chant said her department’s hypothesis was that an unknown person infected the drummer, a woman in her 60s and man in his 70s at Avalon RSL last Friday where Nothing-Too Serious was performing.

Certainly from our point of view, I wouldn’t say it’s exactly how we feel, because it obviously is serious,” Health Minister Brad Hazzard said.

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The woman in her 60s then attended Avalon Bowlo on Sunday, where she infected the woman in her 50s, who in turn infected her partner.

Genomic testing would be completed by Thursday night, Dr Chant said.

Sewage testing last Thursday uncovered no virus fragments in the area, indicating the virus was newly introduced.

“The working hypothesis is that there was an unknown person present at Avalon RSL that was the source of infection, given we’ve got three people at least … that’s potentially the exposure source,” Dr Chant told reporters.

The drummer also played at Penrith RSL on Sunday and at the Kirribilli Club on Monday.

Health alerts have been issued for those two venues, Avalon RSL and numerous others across Sydney’s northern beaches.

NSW Health revealed an additional nine cases had been identified on Thursday afternoon.

The cluster has led WA Premier Mark McGowan to order any arrivals in his state who were in NSW from December 11 to self-isolate until they test negative.

Hundreds of people started queuing at Mona Vale Hospital early on Thursday to get tested for the virus and multiple pop-up testing centres are now in place, including at Avalon, Newport and Frenchs Forest.

Premier Gladys Berejiklian flagged Northern Beaches nursing homes will be locked down until the outbreak is contained, just a week after restrictions eased allowing unlimited visitors.

“We want to get on top of this and don’t want this concerning us in the last few days before Christmas,” Ms Berejiklian told reporters.

There are a number of aged care facilities which are very vulnerable in the northern beaches … we’re recommending no visitors until we identify the source of the infection and feel more confident we have it under control.”

The Pittwater Palms aged care facility is closed to visitors while residents and staff who had been in contact with the infected worker were being tested and were self-isolating.

The latest outbreak has prompted concerns the recently-opened Queensland and WA borders will again shut to people from NSW.

An employee at Pittwater Palms has tested positive for COVID-19. Photo: AAP 

Health Minister Brad Hazzard has implored the two states to avoid “pre-emptive” action, saying NSW would quickly squash the cluster.

Tasmania nevertheless elected on Thursday to ban anyone who had visited the venues on NSW’s health alert list from entering the island state. Those in NSW who are set to travel to Tasmania must also reapply for their e-Travel pass.

The Northern Beaches cases were uncovered after a 45-year-old van driver transporting international air crew members to and from Sydney Airport was on Wednesday confirmed to be COVID-positive.

The southwest Sydney man wore a mask, worked only with air crew members and was not involved in regular taxi services for the public.

The Pittwater Palms employee, aged in her 50s, tested positive for COVID-19 on Thursday and is self-isolating at home.

A spokesperson for Aveo, which runs Pittwater Palms, said no other staff or residents had yet displayed symptoms or tested positive.

The home is among several in the area that have been locked down after confirmation of the latest diagnoses.

The musician whose diagnosis was confirmed on Thursday morning plays in a band that has travelled to RSLs and other clubs across Sydney in recent weeks.

• See the latest information on NSW venue alerts here

NSW Health said the two people confirmed with the diagnosis on Wednesday were at the Avalon RSL on December 11, at the same time as the band member (they are not connected to him).

Thursday’s first two cases were at the Avalon Bowling Club at the same time as the woman in her 60s who was diagnosed on Wednesday.

NSW’s 12 consecutive days without locally acquired cases of COVID in NSW ended on Wednesday after a Sydney airport driver was confirmed to have the virus. Then came the first two in the Northern Beaches, followed by two more on Thursday morning and the fifth in the afternoon.

There has been a rush to get tested in the area following the revelations. People started queuing up at Mona Vale Hospital early on Thursday, and reportedly waited hours.

Mr Hazzard said pop-up clinics would be set up across the Northern Beaches to help meet demand.

“I was getting reports as early as about 9am there were hundreds of people lining up at the Northern Beaches hospital,” he said.

“That is a positive. I want to thank the local community.”

Tweet from @NSWHealth

The latest outbreak has also prompted concerns about whether the recently-opened Queensland and WA borders will snap shut to people from NSW over Christmas.

WA Premier Mark McGowan didn’t rule out re-imposing restrictions.

“If the advice comes back that we need to put up a hard border, then we will,” he said,

Mr McGowan said he realised it would be “very upsetting” but would not hesitate to put up a hard border with NSW “if the medical advice says that is what is required”.

Acting Queensland Premier Steven Miles said the government was not considering fresh border restrictions at this stage, saying the next 48 hours would be important.

Mr Hazzard implored the two states to avoid “pre-emptive” action.

“My blood pressure hasn’t gone up, my pulse hasn’t gone up, we’re just doing what we do in NSW – handling the cases,” he said.

The cases were identified just hours after a 45-year-old driver transporting international air crew members to and from Sydney Airport was confirmed COVID-positive.

-with AAP