Parents of three Sydney school children diagnosed with coronavirus have also tested positive for the disease.

A 14-year-old boy and 15-year-old girl from St Patrick's Marist College were confirmed to have COVID-19. Now the fathers of both students are also confirmed as having coronavirus.

The mother of a 12-year-old girl from Willoughby Girls High who was diagnosed today has also tested positive.

(Nine Digital / Tara Blancato)

While the mother is Iranian , she hasn't recently been to the country, which has a major outbreak.

NSW Premier Gladys Berejiklian visited the Willoughby school last month .

Both schools sent all children home today, and will close tomorrow for cleaning.

Meanwhile this afternoon a third defence force member has been diagnosed.

Parents of three Sydney school children diagnosed with coronavirus have also tested positive for the disease. (Nine)

A boy and a girl, who are both in Year 10 at St Patrick's Marist College in Dundas, in the city's north west, have tested positive for the illness, it has been confirmed. (Facebook)

Coronavirus school closures 'the new norm'

NSW Health Minister Brad Hazzard warned it's a situation which is "likely to be the new norm".

He said any more school cases will also have classes stopped.

"If a child, a staff member, or any other person within that school, is found to have the COVID-19 virus, then effectively a breather will be taken and a day out will be the immediate requirement," he said.

NSW Premier Gladys Berejiklian visited Willoughby Girls High two weeks ago to open a new building. (Willoughby Girls High School)

"Can I also express to the parents and to young people that this particular virus impacts generally young people in a very, very light way.

"But it's not a joking matter.

"Whilst you may be lucky enough to be in a group that's unlikely to have a serious consequence, your mums, your dads, your grandparents, aunties and uncles may be in a different position."

A Year Seven student at Willoughby Girls High School has also this morning been diagnosed with coronavirus. (Nine)

Mark Scott, from the NSW Department of Education, said the latest diagnosed students' close contacts will be investigated.

He told all parents and carers to keep their kids off school if they had any kind of cold or flu-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," he said.

Meanwhile Epping Boys High school reopened today after being closed when a 16-year-old student tested positive for coronavirus last Friday.

There was a mixed response at the Epping school, where most students returned today.

Students return to Epping Boys High School (Nine)

Almost 70 people who had close contact with the diagnosed student stayed away, but classes resumed for the rest after Friday's closure.

One father said he had no worries about the school reopening.

"I'm comfortable. We're relying on the leadership from the school and the health authorities. We cannot overreact with these things," Obeid Hamid said.

Annie Ngu said she was unsure whether to take her son to school.

The mother of a 12-year-old girl from Willoughby Girls High who was diagnosed today has also tested positive. (Nine)

"I am up and down, I don't know. I'm a bit scared. Hopefully it will be fine," she said.

Coronavirus cases in Sydney

Officials are probing links between the new cases, which include two defence force members diagnosed at the weekend and a third today, Ryde Hospital , where a doctor was diagnosed last week.

The schools are nearby the Macquarie Park aged care centre where two elderly people died after a care worker was diagnosed.

The mother of the Epping school student who was diagnosed on Friday worked at the hospital as a nurse.

"Obviously there's a lot of family clusters. It's a bit like detective work trying to find out," NSW Health Chief Medical Officer Dr Kerry Chant, said.

"Obviously we know the greatest risk for transmission is within a family setting."

Dr Chant also urged people "act responsibly" and not to target those diagnosed on social media, as it may deter people going to get tested.

She also said as cases progress, anybody with mild cold or flu symptoms might not even need to be tested, but should just stay at home.

Another new case this afternoon in NSW- a woman in her 30s who has travelled from the Philippines- brings the number in the state to 48.