During a morning press conference, Premier Gladys Berejiklian said of the new cases, all were from known sources, which was "positive".

There were 21,167 tests in the 24 hours leading up to Sunday 8pm.

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Ms Berejiklian reminded people to abide by health advice. If they have been told to stay at home for all 14 days, even if the result came back negative, then they must do so as the virus can develop during the 14-day incubation period.

"Our state continues to be on extremely high alert...and at a critical point," she said. "We have the opportunity to isolate the cases, we have to clamp down, and make sure we reduce the incidence of the virus spreading.

"Unfortunately other places don't have that opportunity.

"We have the ability to keep moving forward.

"So long as you can maintain social distancing you can go about your day but please minimise non-essential travel. Don't go to locations you know there will be large crowds."

The Premier recommended for people to wear a mask if they had to go somewhere and could not guarantee social distancing.

"If we can get through the next few weeks and month then we can have the confidence that we can keep moving forward in NSW," she said.

"But as you know, we are dealing with potential seeding from Victoria, that people were more mobile during the school holidays and restrictions were considerably eased. Those three factors make this a very critical period in our state."

Chief Medical Officer Dr Kerry Chant advised the total number of cases in NSW was 3399 with 96 people being treated by NSW Health with two people in intensive care with one of those cases on a ventilator.

"I want to point out that one of those people in ICU is in their 30s," Dr Chant said. "Young people can still be impacted and we all need to take this seriously."

Of the 20 new cases, there were three people linked to the Casula Crossroads Hotel cluster, bringing the total number of cases to 48.

Eight were related to the Thai Rock restaurant in Wetherill Park mall.

Four people were linked to the Batemans Bay Soldiers Club, which included two diners, one staff member and one contact of a known case.

Four people were returned travellers from overseas who were self isolating and one person had been infected in Victoria and was isolating upon their return to NSW.

Meantime, Minister for Education Sarah Mitchell was looking forward to welcoming back students and staff to schools for term 3 on Tuesday.

She reminded parents to keep their children at home if they were unwell. She said the hand sanitiser stations installed last term would remain in place as well as regular cleaning throughout the day.

In Victoria, another 275 coronavirus cases were recorded and a woman in her 80s has died.

There are 147 Victorians in hospital, 31 of those are in intensive care, Premier Daniel Andrews said on Monday.

Victoria now has a total of 5942 cases since the pandemic began, 2913 of those active.

People aged 12 and up, living in metropolitan Melbourne and the Mitchell Shire will have to wear masks or face coverings from Thursday.

The government announced on Monday students required to be in the classroom from this week would be among the first to receive free masks.

"We already have 1.2 million single use masks and we will be distributing those to all government schools in Melbourne metro and Mitchell shire over the coming couple of days and we will also be providing these single use masks for our Catholic and independent schools," Education Minister James Merlino told reporters.

An order of 1.37 million reusable masks has been made and the Department of Education will be one of the first to receive a batch of them.

"We will have those by early next week and we will be distributing those out to government and non-government schools in Melbourne metro and Mitchell Shire," Mr Merlino said.