Parents have been warned not to send their children to school if they have any symptoms after three students across two Sydney high schools were diagnosed with coronavirus.

Key points: Parents have been asked to pick students up at St Patrick's Marist College and Willoughby Girls High School

Parents have been asked to pick students up at St Patrick's Marist College and Willoughby Girls High School Both schools will be closed on Tuesday

Both schools will be closed on Tuesday There are 46 confirmed cases of coronavirus in NSW

Two Year 10 students at St Patrick's Marist College in Dundas have returned positive results, as has a Year 7 student at Willoughby Girls High School.

Classes at both schools were cancelled on Monday and parents were asked to pick up their children.

Both schools will be closed Tuesday.

NSW Health Minister Brad Hazzard this afternoon said closure of schools linked to coronavirus "is likely to be the new norm".

The ABC understands up to 50 pupils may have had close contact with the two students who were infected at St Patrick's.

The confirmation of new cases comes after Epping Boys High was closed last Friday when a student tested positive.

Mr Hazzard said a rise in student infections was a "cause for concern".

"We want to make sure that we move as quickly as possible to require isolation of others who may have had contact," he said.

"There is a lot of work to be done by NSW Health."

A student in Year 7 at Willoughby Girls High has contracted coronavirus. ( Supplied: Willoughby Girls High School P&C )

Executive director of Catholic Schools Greg Whitby said the relevant staff and students at St Patrick's had been told to self-isolate.

Health officials will develop a plan for infection control at the school's campus tomorrow.

"[We are] asking [students] to be sensible and not panic ... the staff are doing the best they can for the students," Mr Whitby said.

Secretary of the NSW Department of Education Mark Scott warned parents not to send their children to school if they had any symptoms.

"If your child is presenting with symptoms you wouldn't want someone to have if they were sitting next to you on the bus, we don't want those kids in our classrooms at the moment," Mr Scott said.

"The advice to schools, if a child does present with a heavy cold, sore throat, cough, fever or flu-like symptoms, we'll be contacting parents to come and collect their children."

Mr Hazzard said another three cases had been diagnosed with COVID-19 on Monday, bringing the state's total number of infections to 47.

Two men in their 50s and a women in her 30s, who recently travelled form the Philippines, tested positive on Monday.

Catch up on the main COVID-19 news from September 18 with our coronavirus blog.

Mr Hazzard said as the virus spreads it is important to ring your doctor if you feel ill, have recently travelled overseas or have had contact with a known COVID-19 carrier.

The students are enrolled at St Patrick's Marist College in Sydney's west. ( Supplied )

He also implored people to be polite to others in public settings and "remember the motto of mateship".

"This is not a situation for overreaction. We're all in this together, we're doing pretty well."

Last night a man in his 70s was diagnosed after presenting to St Vincent's Emergency Department in Sydney's east on March 6.

The source of his infection is still under investigation as he has not recently travelled overseas.

Another man in his 40s also tested positive after recently returning from overseas.

An 82-year-old man died in Sydney at the weekend after being infected with COVID-19 — Australia's third coronavirus-related fatality.

He was a resident at the same aged care facility in Macquarie Park where another resident, 95, died last Tuesday.

NSW Premier Gladys Berejiklian has called for calm as the number of infections climbs.

"I also want to stress that it's very highly unlikely that if you have the virus that you will have an extreme reaction or an extreme event related to it," she said.

"In the vast majority of cases people recover."

As a result of previously confirmed cases, NSW Health has issued fresh advice to passengers on two recent flights to self-isolate.

Anyone who arrived in Sydney on QR908 from Doha on March 2 and was seated in rows 49, 50, 51, 52 and 53 and anyone on QF02 from Singapore on February 28 seated in rows 31, 32 and 33 should stay at home and seek health advice if needed.

All passengers on the Ruby Princess cruise ship, which arrived in Sydney on Sunday, had to undergo coronavirus testing but were given the all clear last night.