Premier Gladys Berejiklian announced the changes on Thursday morning, saying Greater Sydney residents - including Sydney, the Blue Mountains, Illawarra and Central Coast - will be required to wear face masks in indoor settings and have no more than 20 visitors to their home.

Other restrictions include a dancing and singing ban after another locally acquired case was recorded overnight.

That case was the wife of a Sydney man who tested positive on Wednesday morning.

Chief Health Officer Dr Kerry Chant said she was the only close contact of the man to contract the virus so far. Nine other close contacts returned negative results.

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"We've picked up that person very early in the infection," she said.

Nine cases were detected among people returned from overseas. There were 11,579 tests conducted in the 24 hours to 8pm Wednesday.

For three days from 5pm on Thursday, face masks will be compulsory in indoor settings like shops and public transport, singing and dancing in venues is banned, and no more than 20 people can gather at a home at once.

Questioned on the impact on businesses on Mother's Day, Ms Berejiklian said the response was "proportionate" to the community transmission and "based on health advice".

"If you have a booking for Mother's Day, go to the booking," she said.

"But as a precaution, use your common sense. Make sure if you have the mildest of symptoms you come forward and get tested."

She said it was "business as usual" for communities outside Greater Sydney.

Dr Chant said genomic testing had revealed the likely source of the Sydney man's infection, an overseas case that recently spent time in hotel quarantine.

But she said there was "no direct contact" between the two cases and there was "a gap" authorities were investigating.

Multiple venues were added to the alert list that the Sydney man had visited, along with his wife who contracted the virus.

"Apart from a household member, this man does not appear to have passed it on to anyone," Dr Chant said.

Health Minister Brad Hazzard said the "nasty virus does not show any respect for human kind".

"It could be anyone of us, next, who could get the virus" he said.

He thanked the Sydney couple for using the state's QR code login system and urged people to get vaccinated as soon as they were eligible.

"Any reluctance to get vaccinated should be right out the door," he said.

"Go and talk to your GP. If you don't have a GP, get a GP.

"Get the vaccine as soon as it is available."

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