One more case of COVID-19 has been diagnosed in a man in Sydney's east who visited Bondi Junction, as masks again become compulsory on Sydney public transport.

Masks are once again compulsory on Sydney's public transport after a man in his 50s was diagnosed with COVID-19 after "fleeting" contact with an infected shopper at Bondi Junction last weekend.

"It appears from CCTV cameras that it could have been a very fleeting contact between the infectious person and this gentleman," Premier Gladys Berejiklian said.

The list of Sydney's COVID hotspots has expanded considerably and Ms Berejiklian says masks will be mandatory on public transport in Sydney and The Blue Mountains from 4pm on Friday for five days.

"If you are catching public transport, please, it is compulsory," she told reporters on Friday.

"That is the only thing compulsory between now and Wednesday night. Please make sure if you are catching public transport you are wearing a mask."

Ms Berejiklian urged people who've been at any venue on the expanded NSW hotspot list "that you don't go away anywhere", as the number of people forced to isolate as a result of the Bondi Junction cluster climbed to more than 100.

"We are strongly recommending that if you are a casual contact that you not travel in the next little while," she said.

Casual contacts were warned not to visit hospitals and aged care facilities and visitors to aged care homes will need to wear masks.

The premier also described the spread of the virus as "scary".

"Some people for some reason aren't contagious and some are extremely contagious. The randomness of that is what makes it scary," she said.

"We are recommending that if you go to an indoor (Sydney) venue you wear a mask, whether it is a cinema, hospitality or front-line hospitality workers," she said.

The new case comes after an unvaccinated limousine driver from Sydney's east and his wife were diagnosed and a woman in her 70s was subsequently infected at a Vaucluse cafe frequented by the couple.

Another case - a man in his 40s from Sydney's northwest also tested positive for COVID-19 but NSW Health has not yet concluded if it was false positive, but his household contacts have tested negative.

The woman in her 70s and the man in his 40s are included as new cases in the numbers for the 24 hours until 8pm on Thursday. The man in his 50s will be included in numbers released on Saturday.

NSW Chief Health Officer Kerry Chant said the latest case was a Bondi man who shopped at Myer Bondi Junction on Saturday at the same time as the driver.

He was on the same floor in the same section as the driver and has contracted the same highly infectious Delta strain.

His symptoms began on Tuesday and while infectious he visited venues in Redfern, Newtown, Bondi Junction and Campbelltown and travelled on a train from Newtown to Bondi.

The man's household contacts have tested negative but will now quarantine for 14 days.

Dr Chant said it appeared the Sydney airport limousine driver had passed on the virus with just fleeting contact.

"This indicates that the initial case was highly infectious, as transmission must have occurred through fleeting exposure, noting that the person who caught the infection at the cafe was seated outside and there was no known on-site with the initial case," she said.

Meanwhile, Yates Avenue Public School in northwest Sydney was closed on Friday after a number of its staff attended a COVID-19 exposure site.

The man with the in-doubt case travelled to Canberra on Monday and ACT Health subsequently identified the National Gallery of Australia and a cafe as exposure sites.

The outbreak could mean NSW reintroduces some restrictions ahead of the school holiday period, which begins on June 26.

New Zealand Prime Minister Jacinda Ardern confirmed on Friday that quarantine-free travel will continue with NSW for now.

However, WA and SA have shut the border for those who attended the NSW virus exposure sites.

Victoria also took a similar step, with residents from the City of Sydney, Waverley and Woollahra council areas told to obtain a travel permit, get tested and isolate until they receive a negative result.