He has a number of close contacts who were being tested and were isolating, Chant said.
That includes a man in his fifties from the eastern suburbs who tested positive on Wednesday morning and his wife who tested positive overnight.
Authorities are investigating whether the mystery case is genetically linked to anyone in the quarantine system or cases in other states, and are also checking which virus variant is involved.
With many people expected to gather over the weekend for annual Mother's Day celebrations, the New South Wales state government restricted household gatherings to 20 guests and limited aged care facility visitors to two people per resident.
She also announced temporary public health measures for the greater Sydney region which will come into force from 5pm on Thursday and remain until Monday.
Masks will be mandatory on public transport and at indoor venues.
Ontario to provide an update on the COVID-19 vaccine rollout Wednesday
Immunization Committee were working with perinatal teams to review data on the risks of COVID-19 to pregnant women. Importantly, when those bumps occur, we will pivot and adjust to keep moving forward.
A Sydney man has tested positive for COVID-19, in what appears to be a case of community transmission.
NSW Health Authorities said urgent genome sequencing is now underway to determine the source of infection.
The case, the first local transmission in NSW in more than a month, baffled health officials given the man had no known links to high-risk jobs or people.
Chief Health Officer Dr Kerry Chant said genomic testing linked the latest cases with a returned traveller from the United States at a Darling Harbour quarantine facility.
"We know for a fact there's at least one person, if not more, walking around with the virus not knowing they have it or potentially having attended many events and venues", Ms Berejiklian said.
Players and staff from both Sydney AFL clubs and the NRL's Sydney Roosters are also undergoing testing after the positive case visited Sydney's Moore Park.