A total of 104 people are being treated for COVID-19 in hospital, including 10 in ICU.

More than 28,000 people were tested in the 24 hours to 8pm on Wednesday night.

Ms Berejiklian congratulated those who had come forward for testing, but urged young people especially to limit the number of places they visited.

“If you have the virus and you go out five times a week to different places you could potentially be spreading it to five different locations, and then we have to contract trace everybody,” she said.

"It's not anybody's fault but it's just the nature of the virus. It's so contagious that if you don't know you have it and you are still socialising you have the potential to give it to so many people."

NSW Health’s latest surveillance report showed 16 cases had no known link to other cases or clusters in the past four weeks, confirming COVID-19 was circulating undetected in the community “risking further outbreaks”.

One of these mystery cases was symptomatic for six days before getting tested and isolating, the report revealed.

People in their 20s account for the largest proportion of COVID transmissions in NSW and nationally.

"We're not saying don't socialise or don't go out at all but we are saying please limit your behaviours just in the next few weeks," she said, urging the public to keep up social distancing.

From Friday, access to the state is only allowed through border communities and Sydney Airport.

NSW Health Minister Brad Hazzard said arrangements in border communities would remain as they are, although there is concern about spread of the virus in regional Victoria.

"Our advice was always from our health authorities there was a concern that it may have seeded throughout the regional areas," he told ABC News Breakfast on Thursday.

"I think we're seeing more of that. As a result we have to be really careful for the people of NSW."

He was unable to provide a timeline for when the arrangements might be able to end, but he remained confident a solution would be found to any issues with food supply caused by the new rules and Victoria's latest restrictions.

"In regard to our food supply ... we'll find ways around it, the governments around the country will find ways around it," he said.

"We'll work together to make sure food supplies still get through."

Asked about why the state government has stopped short of making masks compulsory, Mr Hazzard said there were a range of reasons health officials did not believe it was necessary.

"One is that if you make it a mandatory requirement there will be massive demand," he said.

NSW Opposition Leader Jodi McKay is pushing for compulsory masks where the state's health department is currently "strongly recommending" they are worn: on public transport, at supermarkets, shopping centres and in places of worship.

In Newcastle, people who visited three pubs have been directed to immediately self-isolate after it was confirmed a man in his 20s who later tested positive for COVID-19 had attended last week.

Hunter New England Local Health District is urging anyone who attended the following venues at the specified times to immediately self-isolate and seek testing:

• The Bennett Hotel at Hamilton on Friday between 5.30pm and 10pm
• The Greenroof at Hamilton on Friday between 10pm and midnight
• Hopsmith Sports Bar, Wests New Lambton on Sunday from 5pm to 7.30pm

Other locations attended by the man include:

• The Queens Wharf Hotel between 9.30pm and 11pm on Saturday,
• The Sydney Junction Hotel between midnight and 1.30am on Saturday and;
• McDonald Jones Stadium on Sunday, during the Newcastle Jets game.

People who attended these three venues do not need to self-isolate but should monitor for symptoms.

The case is a close contact of the teenager who attended St Pius X High School, causing that school to close today, Hunter New England Health said.