There has also been a man in his 40s diagnosed overnight, who presented at the Mount Druitt Emergency Department late on Saturday night.
He was moved to an area of the Emergency Department reserved for suspected COVID-19 cases, and - when a positive result was returned on Sunday night - the patient was transferred to Westmead Hospital for treatment.
Urgent investigations and contact tracing are underway. Anyone determined to be at risk of possible exposure is being contacted and advised to get tested and isolate.
The ED is undergoing deep cleaning and will soon be operating as normal, NSW Health said.
Premier Gladys Berejiklian said there were not yet any direct links from this case to others, but said "contact tracers do a great job of finding those cross-over points of contact".
There were 18,073 tests reported in the 24 hours period, compared with the previous day's total of 23,763.
NSW Chief Health Officer Dr Kerry Chant said it was "critical" that numbers hit around the 30,000 test mark.
Eight cases were also recorded in returned travellers.