Premier Gladys Berejiklian says COVID-19 is "still lurking" in the community, despite continued low numbers of new cases in recent days.
There were just three cases found in the state in the 24 hours to 8pm on Sunday night, and two of those were acquired overseas and detected in hotel quarantine.
The third was linked to a close contact of a previous case - a health care worker at Liverpool hospital - NSW Chief Health Officer Dr Kerry Chant said.
These cases were identified among more than 20,000 tests - a high number on a weekend - as the dstate passed the two million test milestone.
Despite these positive numbers Ms Berejiklian said that health officials believed there were still strains of the virus circulating undetected, especially in the South Western Sydney area.
She said the health department was aware of "around 15" cases that had not been linked to known clusters in the last six or seven weeks.
"[This] tells us that there are still undetected sources of the virus in south-western and Western Sydney and potentially people walking around with the virus giving it to others without knowing," she told reporters.
"That's why it's really important for people to come forward and get tested."
She said some cultural groups may be hesitant about getting tested, or contacting public health officials, and said she wanted to break the stigma of getting tested.
"Dr Chant, the [health] minister and I remain concerned about the community transmission still lurking," she said.
"It's really important to make sure we're not missing any of these strains. We're just making sure, we're covering all the bases."
Ms Berejiklian said she wanted to recognise some of the community leaders who had helped break the stigma of getting tested, and said people from all sections of society had been tested in recent months.
"I'm one of them and I'm sure many of you are as well, and you might have to get tested more than once," she said.
"If we continue on the path we're on, NSW will fare very well."
Dr Chant also urged people to come forward for more testing, highlighting that it was always free, regardless of whether people were eligible for Medicare.
NSW Health also issued an alert to say that people who attended the following venues where a previous case visited at the times listed should monitor for symptoms, no matter how slight, and immediately isolate and get tested if they appear.
- Auburn Ramadan Pharmacy from 1.30-2.30pm on Wednesday 19 August
- Westfield Shopping Centre Burwood from 6-7pm on Thursday, 20 August, including Kmart and Woolworths from 6.30-7pm
- Service NSW Burwood 2.30-3pm on Friday 21 August.
There are currently 89 COVID-19 cases being treated by NSW Health; seven are in intensive care and five are ventilated. 84 per cent of cases being treated by NSW Health are in non-acute, out-of-hospital care.