New South Wales Premier Gladys Berejiklian announced today masks are compulsory in indoor venues and no gatherings larger than 20 people are allowed in residential homes for the next three days.
Chief health officer Dr Kerry Chant said that while ten close contacts of the man who tested positive yesterday have been identified, the "missing link" in the case is still out there.
Genomic testing shows the sequence from the new case in the community, an Indian variant, matched with an overseas case, who had travelled from the US and returned to Australia on April 28.
"We can't find any direct link between our case, so what we're concerned about is there is another person that is as yet unidentified that infected our case," Chant said.
New Zealand officials say they remain in contact with their Australian counterparts on the case.
"Anyone in New Zealand who has been at any of the locations of interest at the specified times should contact Healthline on 0800 358 5453, self-isolate and be tested as soon as possible," the ministry said.
"Individuals in Australia who were at a location of interest at the specified times should follow New South Wales health advice regarding isolation and testing, and should not travel to New Zealand. The ministry has requested airlines communicate this message to anyone before flying to New Zealand from New South Wales."
The public health assessment of the risk remained low, the ministry said.
"We expect to be able to provide a further update on the situation in Sydney later today."
Brisbane Airport green zone breach
Thirty-two people in New Zealand have been advised to isolate at home until they receive a negative result from a day 5 test.
So far 22 people have returned a negative result.
Three new cases were identified in New Zealand yesterday - all of them in managed isolation facilities in Auckland.
Because of the new positive Covid case in Sydney, there were concerns that they may pause quarantine-free travel between Sydney and New Zealand or other parts of Australia.
Kiwi health officials are continuing to monitor the Sydney case but say the risk is now and they do not recommend any change in quarantine travel between New Zealand and Australia.