The number of confirmed cases of coronavirus in NSW almost doubled over the weekend to reach 171.

NSW Health Minister Brad Hazzard says there have been 37 new cases in the 24 hours to 11am on Monday which he described as a "dramatic increase".

There are currently 1,282 cases under investigation, with 25,500 people tested to date.

READ MORE Victoria, ACT declare states of emergency to deal with coronavirus crisis

"The number of people who are being impacted in our community is still relatively small," Mr Hazzard said in Sydney.

"Having said that, it is starting to look as if there will be a fairly substantial ... exponential increase in numbers over the next few weeks."

The minister said it was not known how 17 of the NSW cases had contracted the virus.

There were 92 cases in the state on Friday evening.

AAP

Flights coming from the United States in the middle of last week had passengers who tested positive for COVID-19, Mr Hazzard said.

NSW chief health officer Dr Kerry Chant says people who have travelled or are contacts of confirmed cases are being prioritised for testing along with people suffering respiratory issues.

"It is important we keep the testing for people with symptoms that are at high risk," Dr Chant said on Monday.

"If we're swamped with a lot of testing that is of a low value ... it means there could be delays for us finding cases that are actually putting the community at risk."

Some private schools will start teaching students online to limit the spread of coronavirus while harsh penalties will be imposed on people and businesses who fail to comply with a ban on mass gatherings.

READ MORE Woolworths to hold daily elderly and disability-only shopping hour amid panic buying

Mr Hazzard has issued an order forcing the immediate cancellation of public events with more than 500 people.

Individuals who fail to comply could face up to six months in prison, a fine of up to $11,000, or both. Corporations face even harsher fines.

Premier Gladys Berejiklian has urged people to be careful.

"Don't just think of yourself or your own family, but you could be unintentionally infecting and causing the death of so many other people," she said in southern NSW on Monday.

"I want everyone to brace themselves. It's not a time to be complacent or reckless or think that it's not going to affect you."

Schools from Monday are adopting social distancing measures including cancelling assemblies, excursions and travel, as well as some events and conferences in a bid to slow the spread of COVID-19.

The Association of Independent Schools of NSW said some of its schools will start teaching students remotely.

There are about 500 private schools across the state with some to teach students online from this week while others will set tasks and assignments via email.

St Christopher's Catholic Primary School in Panania in Sydney's southwest closed on Monday after a person who attended an event hosted by the school tested positive the disease.

READ MORE Australia's coronavirus death toll rises to five as new border restrictions come into force

Sydney Catholic Schools executive director Tony Farley said the professional development event was held on March 12 and the infected person was not a teacher at the Panania primary school or any other Sydney Catholic school.

The school will undergo a thorough "hygiene sweep" on Monday and will reopen on Tuesday.

NSW Labor leader Jodi McKay on Monday said she had written to the premier advising her it would be inappropriate for state parliament to sit next week.

AAP

Meanwhile, the University of NSW said a student from its Australian Graduate School of Management on Monday tested positive.

The student has not been on campus or been in contact with any students or staff, a UNSW spokeswoman told AAP in a statement.

There are more than 300 coronavirus cases across Australia.