Health authorities are scrambling to track down the mystery link between a returned traveller who tested positive to Covid and a man from Sydney's east who became infected.

NSW chief health officer Dr Kerry Chant revealed on Thursday she was 'concerned' there was someone in the community who unknowingly had Covid after contact tracers were yet to identify how the virus spread between the two people.

Genomic testing determined the man's Covid infection is linked to a traveller who returned from the US - but authorities are still unsure how he contracted the virus.

'Our new case matched with an overseas case, who had travelled from the US and went to the Park Royal at Darling Harbour,' Dr Chant said.

'What we're concerned about is there is a missing link.

'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.'

The returned traveller tested positive on day one of his mandatory hotel quarantine stay and was moved to a health hotel on April 28.

The man returned a positive test on Wednesday. His wife has since tested positive.

'As we get further information over the next couple of days, we're hoping that we can really ascertain the missing link and determine that we've tracked down any chains of transmission,' Dr Chant said.

She added health officials are reviewing CCTV footage to see how the returned traveller may have infected the man from Sydney's east.

They man and his wife were the two cases announced by officials on Thursday, prompting premier Gladys Berejiklian to introduce a raft of new restrictions.

Masks will be mandatory indoors and dancing will be banned expect at weddings across Sydney and surrounds including Wollongong, Central Coast, and the Blue Mountains.

Household gatherings are also limited to just 20 people and drinking while standing up is not permitted at indoor venues.

The new restrictions will be enforceable from 5pm on Thursday and will stay in place over the weekend until 12.01am on Monday.

'We know that at least one person with the virus has been going around their business and we haven't found them yet,' Ms Berejiklian said.

'We don't know where they've been. We don't know if they've been to major events. We don't know who they've sat next to. We know for sure that someone with the virus has been moving around the community.'

Ms Berejiklian reiterated the importance of the state having a 'proportionate response' until the missing link between the infected man and the returned traveller was found.

Health authorities also identified two new venues of concern as part of its ongoing investigations.

One of the Covid-positive cases dined at ?Fratelli Fresh in Westfield Sydney on Tuesday from 1.15pm to 2.15pm.

Bondi Trattoria on Campbell Parade has also been listed as a potential exposure site after it was attended by a case on Thursday, from 12.45pm to 1.30pm.

Anyone who attended the two venues at the times specified is asked to get tested and isolate until NSW Health provides further advice.

Anyone who attended one of the following venues at the times specified is asked to immediately get tested and isolate until NSW Health provides further information.

Tuesday April 27, 1.15pm - 2.15pm

?Thursday April 29, 12.45pm - 1.30pm

District Brasserie, 2 Chifley Square in Sydney

Friday April 30, 11am - 11:45am

HineSight Optometrist, Sofitel Sydney Wentworth - P2/61-101 Philip Street in Sydney

Friday April 30, 12pm - 1pm

Barbetta, 2 Elizabeth Street in Paddington

Friday April 30, 1:30pm - 2:30pm

Friday April 30, 6pm – 8pm

Friday April 30, 8.45pm – 11pm

Saturday May 1, 1pm –1.45pm

Saturday May 1, 1pm – 1.45pm

Saturday May 1, 2pm – 3pm

Saturday May 1, 4pm – 5pm

Saturday May 1, 4.30pm – 5pm

Sunday May 2, 3pm – 4pm

The Stadium Club, Moore Park - Entertainment Quarter, 122 Lang Road

Monday May 3, 11.30am - 12.30pm

Azure Cafe, Moore Park - Entertainment Quarter, Shop 5a

Monday May 3, 12.30pm - 1pm

The Royal Sydney Golf Club, Kent Road, Rose Bay

Monday May 3, 12.30pm - 1pm

Rug Cleaning Repairs Hand Rug Wash Sydney, 8/52-54 Sydenham Rd, Brookvale

Monday May 3, 12.30pm - 1pm

Alfresco Emporium, 2021 Pittwater Rd, Collaroy

Tuesday May 4, 1pm - 1.30pm

Smith Made, 14/28 Roseberry Street, Balgowlah

Tuesday May 4, 2.30pm - 4pm

Chemist Warehouse, 459-463 New South Head Road, Double Bay

Tuesday May 4, 3.45pm - 4pm

Woolworths, Kiaora Lane and Kiaora Road, Double Bay

Tuesday May 4, 4.05pm - 4.15pm