New South Wales has recorded one new community case of COVID-19, a day after a man from Sydney's eastern suburbs also tested positive for the virus.
The new case is the wife of a man in his 50s who visited while infectious. Nine other close contacts have returned negative tests overnight.
Health authorities have been unable to determine the source of his infection, sparking fears the virus could be circulating undetected in the community.
"While we remain on high alert, pleasingly, at this stage, nobody else has been identified as being a positive case," NSW Premier Gladys Berejiklian told reporters on Thursday.
She also announced temporary public health measures for the greater Sydney region which will come into force from 5pm on Thursday and remain until Monday.
The restrictions include a 20-person limit for at-home gatherings and the reinstating of compulsory face masks in indoor venues, including public transport.
Singing and dancing at indoor venues, with the exception of weddings, will also be outlawed for the next three days.
On Thursday afternoon, New Zealand announced a pause of quarantine-free travel with NSW for 48 hours.
It's the second pause to hit the trans-Tasman bubble since it came into effect last month, after travel from Western Australia was stopped last week.
Queensland and WA have also announced they will send anyone who has visited an exposure site in Sydney into hotel quarantine from Friday morning.
Genomic testing overnight revealed the new infections were linked to an overseas case who had travelled from the United States and undertook their mandatory quarantine across two facilities in Sydney.
No direct link has been discovered between the returned traveller and the Sydney man, NSW Chief Medical Officer Kerry Chant said.
"So what we're concerned about is there is another person that is as yet unidentified that infected our case," she said.
Staff working at the two quarantine facilities, the Park Royal at Darling Harbour and the Sydney Health Accomdation, are being tested as health authorities rush to determine how the virus leaked into the community.
Dr Chant also announced a number of new at-risk venues, including Fratelli Fresh at Westfield Sydney, F5 on Pitt Street in the CBD, and the Bondi Trattoria restaurant.
Anyone who was at the venues at the times listed on the NSW Health website are being urged to isolate and get tested.
"We know for a fact there's at least one person, if not more, walking around with the virus not knowing they have it or potentially having attended many events and venues," Ms Berejiklian said. "That's why it's important for us to have a proportionate response."
Earlier on Thursday, it was revealed after he attended the same CBD restaurant as the man on Friday.
He has since returned a negative COVID-19 test.
Players and staff from both Sydney AFL clubs and the NRL's Sydney Roosters have also undergone testing after the positive case visited Sydney's Moore Park.
