NSW has reported just four new cases in the 24 hours to 8pm on Sunday, which were found among just over 10,000 tests.
One is a returned overseas traveller in hotel quarantine and three are locally acquired.
All three locally acquired cases were healthcare workers, one from Concord Hospital and two from Liverpool Hospital.
Read more: Victoria reports nine new COVID-19 deaths
These workers were diagnosed following investigations into a case reported on September 5, who worked two shifts at the emergency departments of both hospitals while potentially infectious.
The additional cases worked at Concord ED on September 1 from 7pm until 7am while potentially infectious, and at Liverpool ED on September 2-4.
NSW Health said the three latest health workers reported having no symptoms while at work and wore personal protective equipment while caring for patients.
In addition to today's case numbers, a visitor to a patient at Concord ED on September 1 has tested positive. This case will be reported in tomorrow's numbers.
Patients considered close contacts and all staff working at Concord and Liverpool EDs at the same times as the positive cases are being isolated and tested.
Investigations into the source of these infections are ongoing.
Premier Gladys Berejiklian said she was pleased with the low daily numbers, but warned one of the new cases "actually got the disease while they were wearing a mask".
She said NSW Health were investigating whether the person contracted it from touching a surface, and said it was a good reminder that "masks are important, but they are only one piece in the line of our defence".
Dr Kerry Chant said NSW Health was looking into how the transmission occurred, and whether it was passed along by someone inadvertently contaminating their hands and then touching their face.
"People should be assured these hospitals are safe places to go to," she said.
Meantime, in Victoria there were 41 new COVID-19, which was the lowest daily total since June 27. Nine more Victorians died from the virus.
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