Health authorities have discovered another new Covid-19 case in Sydney, just hours after an eastern suburbs man tested positive to the virus.

It has raised questions about whether this could spark a new wave of restrictions for Greater Sydney.

Authorities are scrambling to determine how the man in his 60s contracted the virus. He has not been overseas in recent times but does work as a driver, which includes transporting international flight crew.

Just before 10pm on Wednesday night, NSW Health revealed a household contact of a case reported earlier in the day had tested positive to Covid-19.

The man had a Covid-19 saliva test on Tuesday, June 15, with the positive result confirmed yesterday in a PCR test.

"Urgent investigations into the source of the infection and contact tracing are underway, as is genome sequencing," NSW Health said in a statement.

Multiple exposure sites have now popped up across Sydney, with contacts being urgently contacted and asked to isolate.

The state's last locally acquired Covid-19 cases were in May, when a husband and wife, who were also from the eastern suburbs, tested positive to the virus.

The infections prompted Premier Gladys Berejiklian to introduce a range of restrictions for Greater Sydney.

Genomic sequencing showed the likely source of the couple's infection was from a returned traveller from the US.

However, authorities were unable to find the "missing link" between these cases, which was one of the main reasons behind the tightening of restrictions.

Authorities are now scrambling to find the source of infection for this new eastern suburbs case and track down any chains of transmission that may be going undetected in the community.

Burnet Institute epidemiologist Professor Mike Toole said the next 24 hours would hopefully provide critical information relating to this case.

He told ABC's Patricia Karvelas that whether or not restrictions will be brought in and how long they are in place will depend on whether any more positive cases are discovered.

Professor Toole said if there is only a small rise in linked cases then authorities would likely "go fairly easy on restrictions".

"But you get a number of cases arising from say, more than one of those exposure sites, and they may not be linked to each other, then I think we would have to consider stricter lockdowns," he said.

"We know once you see this pattern getting a little out of control, you must go in quickly. And that usually leads to a short lockdown rather than the kind of dragged out lockdowns you have seen in places like the UK."

Professor Toole also noted the man visited a lot of sites while potentially infectious, saying Victoria's outbreak has shown how some of the more infectious variants can "spread very quickly through just casual contact one of those environments".

Hotel quarantine investigated

This follows a late night alert from NSW Health on Tuesday, sparking fears of a breach in one of Sydney's quarantine hotels after it was discovered the virus had jumped between three returned travellers.

Authorities are investigating the source of a Covid-19 case diagnosed in hotel quarantine which has an identical viral sequence to two cases who were staying in an adjacent room.

"It is currently unclear how and where transmission occurred from a couple to another returned traveller who were all staying on the fourth floor of the Radisson Blu quarantine hotel," NSW Health said.

All three cases have identical viral sequences of the Alpha Covid-19 strain, formerly referred to as the "UK variant".

The couple were asymptomatic and tested positive to the virus on June 3 during the second day of their hotel quarantine stay.

The other traveller returned a negative Day 2 test on June 3 before subsequently developing symptoms and testing positive for Covid-19 following on June 5.

All three cases arrived into Sydney on the same flight from Doha on June 1 and stayed in adjacent rooms in the quarantine hotel.

"At this stage we still don't know if it happened on the plane, transporting from the plane to the accommodation, at the accommodation or whether it was just a coincidence," NSW premier Gladys Berejiklian said.

- by Ally Foster, news.com.au