The state reported no new locally transmitted cases in the 24 hours to 8pm on Saturday, NSW Health said.

There were 13,721 tests reported in this period, compared with 14,697 tests in the previous 24 hours.

The three new cases in hotel quarantine bring NSW's total to 4273.

NSW Health is treating 63 cases, with 95 per cent of them in non-acute, out-of-hospital care.

One patient is in intensive care and does not require ventilation.

"We continue to urge anyone with even the mildest of symptoms to seek testing immediately, then remain in isolation until a negative result is received," NSW Health's Dr Michael Douglas said on Sunday.

The source of recent cases in Moss Vale remains under investigation, Dr Douglas said, with the department again pushing for people in southwest Sydney to get tested.

He said this message particularly applied to people in and around Leppington, Hoxton Park and Prestons, as well as in the Southern Highlands region, including Moss Vale, Mittagong and Bowral.

Extra testing capacity has been made available in these regions, Dr Douglas said.

"The process of becoming infected and infecting others often occurs silently, invisibly, and inadvertently," he said.

"It is evident that the virus has an ability to readily spread from person to person. Some who are infected with the virus may not be aware that they have the infection, where symptoms are mild or not present at all."

Dr Douglas said to ensure NSW could control the virus, people must step forward for testing if there were any concerns at all regarding symptoms.

He also said it is "very likely" COVID-19 would continue to circulate in the community, including among people with mild or no symptoms, during the upcoming holiday period.

"It is important we all continue to take advantage of outdoor venues and maintain the COVID-safe practices we have become used to," Dr Douglas said.

"Contact tracing to contain the transmission of the virus depends on businesses having effective COVID safety plans and customer sign-in processes.

"The recent cluster in southwestern Sydney highlights the importance of these measures, and we urge businesses and patrons to do their part."

NSW Health has also called on people in the Rouse Hill area to get tested if showing even the mildest symptoms after the state's sewage surveillance program detected traces of the virus.

Fragments of the virus that causes COVID-19 have been detected in samples taken on November 5 from the sewerage system that drains parts of Quakers Hill, Castle Hill, Annangrove, Kellyville, Box Hill, Kenthurst, Glenhaven, The Ponds, Rouse Hill, North Kellyville, Kellyville Ridge, Beaumont Hills, Stanhope Gardens, Baulkham Hills, Glenwood, Bella Vista, Parklea, Acacia Gardens and Norwest.

The positive sewage result can be due to shedding of the virus by someone who may have previously had the illness, with the virus 'shedding' through their system for up to six to eight weeks later, NSW Health said.

Australian Associated Press