NSW health authorities are urgently trying to track four people who may have been exposed to COVID-19 after three returned travellers at a Sydney quarantine hotel contracted the same South African strain of the virus.
It comes as Western Australia also reports two COVID-19 infections contracted in hotel quarantine.
On Wednesday night NSW Health said two of three people diagnosed with the virus were family members who had stayed in connecting rooms on the 10th floor of the Mercure Hotel in Sydney earlier this month, while the third person was staying in another adjacent room.
NSW Chief Health Officer Kerry Chant said on Thursday up to 40 returned travellers and an unknown number of staff may have been exposed to the virus and health authorities were still working to track down at least four people.
"We have managed to contact 36 of those individuals," she said.
"We are urgently escalating contact with the remaining four."
A number of the hotel guests had since gone to other states and territories and authorities there had been alerted.
Staff potentially exposed will also have to self-isolate.
Anyone who stayed on the 10th floor of the Mercure between 7 April and 12 April, is being directed to get tested and self-isolate until 14 days after they left quarantine.
The three infected returned travellers arrived on the same flight on 3 April.
However, Dr Chant said transmission at the airport or plane was extremely unlikely, as all parties tested negative on day two in quarantine.
Health authorities are investigating how a breach may have occurred, but Dr Chant said the detection showed NSW's systems are doing their job.
"We have put in range of measures at the borders to protect us from incursions and it's been pleasing to see that the whilst we have detected these incursion events, they really haven't then led to further transmission," she said.
Health authorities said that a separate case of transmission between hotel rooms of the Adina Apartment Hotel at Town Hall was ongoing, and all contacts located so far have tested negative.
The state recorded no new locally acquired cases on Thursday, while seven new cases were acquired overseas.
Meanwhile, Premier Gladys Berejiklian has announced the state's first mass vaccination hub at Sydney Olympic Park in Homebush will be open for business by mid-May.
"We're expecting this hub to be able to dispense around 30,000 vaccines every single week," she said.
"This has not been done ever, to my knowledge, in NSW," Health Minister Brad Hazzard said.
GPs will remain the first port of call for those receiving the AstraZeneca jab, but those who want to be vaccinated in a state facility or don't have a GP will be able to get vaccinated at the hub.
Those with underlying health issues are particularly encouraged to go through their GPs.
The Homebush hub will predominantly administer Pfizer vaccines, which are trickier to store and distribute because they have to be stored at extremely low temperatures.
Some other vaccine sites across the state that have been distributing AstraZeneca jabs will be converted to allow them to dispense the Pfizer vaccine.
More than 180,000 COVID vaccinations have already been administered by NSW Health.
Two contract COVID-19 in Perth hotel
Meanwhile in Western Australia, two returned travellers have contracted COVID-19 in hotel quarantine, with genome sequencing confirming transmission occurred at the Mercure Hotel Perth.
"The virus was transmitted in hotel quarantine at the Mercure Hotel Perth, as two sets of guests, in rooms opposite each other, had the same sequence of virus - despite arriving from different countries at different times," the WA Health Department said in a statement on Wednesday night.
Other guests who stayed on the same sixth floor at the same time were previously released from the hotel after testing negative.
"These guests will be re-tested and directed to self-isolate until cleared by the public health team," the department said.
People who were in rooms immediately adjacent to the infected people will be asked to self-quarantine for 14 days.
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