One of the four new COVID cases recorded since 8pm on Wednesday has come from an as yet unknown source.
"The first locally acquired case is from south western Sydney, and the source of their infection is still under investigation," a NSW Health spokesperson said.
"The other three contacts are household or close contacts of this case. One of these cases was first reported in a public health alert yesterday.
"Two of these new cases were students who attend the Hoxton Park campus of the Malek Fahd Islamic School. The school will be cleaned and will be non-operational for 14 days."
One extra case was also reported in an overseas traveller in hotel quarantine, bringing the total number of cases in NSW to 4222.
Fragments of COVID-19 were detected in sewage at treatment plants in West Camden on Wednesday, as part of the state's ongoing sewage surveillance program.
While the detection could be linked to known cases, the spokesperson said there was concern there may be other active cases in the local communities in people who have not been tested.
"To fight the pandemic, anyone with even the slightest of symptoms must get a COVID-19 test the day they develop," the spokesperson said.
"People need to be vigilant for any symptoms of illness, and immediately isolate and get tested should even the mildest of symptoms like a cold appear. After testing, you must remain in isolation until a negative result is received."