People in Sydney’s east have been asked to restrict their movements and wear masks on public transport after confirmation of a third community COVID case in less than 24 hours.

A woman in her 70s who visited a cafe listed as an exposure site on Wednesday was confirmed with the virus on Thursday.

NSW Health is also investigating a potential fourth community case – a man in his 40s in the Baulkham Hills area who tested positive for COVD on Thursday morning. Premier Gladys Berejiklian said it was possible his was a false positive or historic infection.

Both of the new cases follow the Sydney airport driver in his 60s who became NSW’s first community infection in more than a month on Wednesday. State health authorities say he has the Delta strain of the virus.

The driver’s wife was also diagnosed with the coronavirus on Wednesday.

“At this stage, we’re all on high alert,” Ms Berejiklian said.

“In eastern Sydney and greater Sydney, we’re asking everybody to be extra careful. If you’re in the eastern suburbs, unless you absolutely have to attend a large gathering, unless you absolutely have to engage in activities of a social nature in the next few days, we ask everybody to refrain from that, to be extra careful, to make sure you hand sanitise and socially distance.

“If you’re catching public transport, we recommend people wear a mask.”

Tweet from @NSWHealth

Chief health officer Kerry Chant said the airport driver – who transported international air crews as part of his job – has a Delta strain not seen previously in Australia. It is a match for a strain from the US.

The woman in her 70s who is the third case is thought to have caught COVID at the Belle Cafe in Vaucluse, which was listed as an exposure site by NSW Health.

“That person had attended the Belle Cafe at the same time as the driver on June 13. We consider all people who were at that cafe at the same time as the driver to be close contact and they must all be tested and isolate for 14 days after that visit,” Dr Chant said.

“Because we’ve had transmission at that cafe, we’re urging everyone to take the request for testing seriously. Please present now.”

• See an updated list of NSW exposure sites here

Ms Berejiklian said the confirmed local cases – the first since early May – were a reminder of the need to be vigilant.

“We’ve been through this before. We can’t be complacent,” she said.

“We know that two can become 20 very quickly.”

Other potential exposure sites include Myer and David Jones in Bondi Junction, Harry’s Coffee and Kitchen in Bondi Junction and cafes in Vaucluse and North Ryde.

Extra pop-up testing clinics have been set up across eastern Sydney and Ms Berejiklian urged people in areas visited by this week’s cases to get tested.

The Western Australian and Tasmanian governments have told anyone who has recently returned from NSW to check the list of exposure sites, and get tested and isolate if they have been to any of the listed locations. Queensland is also requiring anyone who has been to an exposure site to go into quarantine.

Investigations are also underway into a potential hotel quarantine breach at Sydney’s Radisson Blu hotel.

-with AAP