NSW has gone six days with no community transmission of COVID-19, but Premier Gladys Berejiklian has again pleaded for vigilance during the school holidays.
One locally acquired case was included in the records in the 24 hours to 8pm on Wednesday, however Ms Berejiklian confirmed the case was an old infection.
The man in his 50s from southwest Sydney was re-tested as a pre-work assessment and was found to have "remnants" of COVID-19, she said.
"What it demonstrates is that our suspicions of the disease lurking in the community without us getting every single case has proven to be the case," the premier told reporters on Thursday.
NSW Health said a repeat negative swab test and positive serology results showed the man had an immune response to the virus and the case was most likely acquired in southwestern Sydney about July.
Two other cases were recorded from 13,072 tests - both returned travellers in hotel quarantine.
In the past nine days, the state has recorded only one new locally acquired case.
Ms Berejiklian has implored people to remain vigilant and to keep coming forward for testing even with the mildest symptoms.
"We can't let complacency get in the way of us continuing to make sure we do everything we can to keep the virus at bay," she said.
"It's important for us to keep those testing rates high - especially as it's the school holidays and especially as from today officially some restrictions we announced last week come into effect."
About 150,000 more NSW residents are now free to travel to Queensland after the Sunshine State's border bubble shifted 100 kilometres further south.
From Thursday residents in the Byron Shire, Ballina, the city of Lismore, Richmond Valley including Casino and Evans Head, Glen Innes and 41 other NSW postcodes have been added to the Queensland-NSW border bubble.
Queensland residents are also able to visit far northern NSW.
For months people from NSW coming to Queensland from outside the old border bubble were required to quarantine for 14 days at their own expense.
Ms Berejiklian is adamant all border restrictions should be removed but Queensland Premier Annastacia Palaszczuk says that will not happen until NSW has 28 days without a case of locally acquired COVID-19.
NSW Health is treating 50 COVID-19 cases, including three in intensive care, none of whom are being ventilated.
© AAP 2020