Covid-19 cases at Cherrybrook's Tangara School for Girls have been linked to a religious retreat held in Bargo.
There were 18 new cases of the virus detected in NSW in the past 24 hour period, with the vast majority of those contracted through community transmission.
Fifteen of the new cases were acquired locally, one is a returned traveller in hotel quarantine and two people who caught the virus in Victoria.
Premier Gladys Berejiklian has warned further measures could be introduced if the community does not increase its use of masks and distancing compliance as the number of outbreaks and cases not linked to other sources continuing to grow.
Ms Berejiklian reminded residents that NSW was "on high alert" and said she wanted to see an higher uptake of distancing measures.
"Whether it is a school extra-curricular activity which shouldn't occur, whether it's the recommendation for people to wear masks when they can't guarantee social distancing on public transport or in supermarkets, or whether it's the way in which we've asked businesses to approach a Covid-safe environment, compliance is absolutely critical," she said.
"I want to see a greater uptake in all those categories. If we don't see a greater uptake in the next little while, we will consider further measures in which we can increase that uptake.
"Some business are doing so well with Covid safe... and other businesses seem to be applying a partial model."
NSW Health has not listed Bargo as an area of concern as the Tangara cluster continues to grow.
Multiple media outlets are reporting that several students who tested positive for Covid-19 attended a retreat coordinated by the Opus Dei study centre Eremeran which was held at the Bargo Conference Centre on Hambridge Road.
The school released a statement to the press which said Tangara did not organise the retreat and was not aware that the study camp would be going ahead.
In a statement to the AAP Eremeran confirmed a recent retreat was attended by five high school girls in year 10 and 11.
"We are assisting NSW Health in their endeavours to ascertain whether this may have contributed to the outbreak," the statement said.
Yesterday Ms Berejiklian said out-of-school activities should not be running at this time.
"Extra-curricular activities, those excursions and overnight things which you would ordinarily do, are not acceptable during a pandemic," she said.
"Every organisation, every entity needs to abide by the Covid Safe plans because otherwise we risk having a surge in numbers and we risk having new clusters."
NSW Chief Health Officer Dr Kerry Chant said the Tangara School for Girls outbreak had grown to 19, and said tracing this outbreak was "challenging" for health officials.
"Over the past six weeks we have seen a number of locally acquired cases where the source has not been able to be identified," Dr Chant said.
"Those are mainly occurring in western Sydney and south-western Sydney."
She said people in those communities should come forward for testing with even the most minimal symptoms, and warned everyone to be on high alert.
"It's also time to reflect on our behaviours," she said, saying mask wearing and physical distancing was essential.
The source of the Tangara school outbreak is still unknown.
For the full list of Covid-19 testing locations, visit: health.nsw.gov.au.