Two Sydney schools have temporarily closed as the number of students testing positive to COVID-19 is rising.

Three secondary school students from two separate Sydney schools are among six new cases of coronavirus in NSW, with concerns the two year 10 students from St Patrick’s Marist College in Dundas may have come in contact with up to 50 people.

A year 7 student from Willoughby Girls High School and her mother have also been diagnosed with the virus.

In Victoria, three new coronavirus cases have been confirmed in Victoria, taking state’s total to 15.

The latest confirmed cases takes the number of Australians testing positive to the virus to 76.

Australia’s chief medical officer Professor Brendan Murphy told reporters the majority of people who contracted the disease experienced a very mild illness.

“Many of the people recently diagnosed are in home isolation because they’re so well.

“We’ve really only had three very severe cases – the ones who have died. So the vast majority of cases in Australia have had a mild illness,” he said.

NSW Health Minister Brad Hazzard said it was a “cause for concern”.

“Obviously when you have cases at a school, it is cause for concern because we want to make sure that we move as quickly as possible to require the isolation of others who might have had contact with the confirmed case,” Mr Hazzard told reporters on Monday.

Catholic Education Diocese of Parramatta executive director Greg Whitby said parents of students at the co-ed St Patrick’s Marist College have been asked to pick up their children from the school if possible.

“We couldn’t close the school because we have students who may or may not have access to childcare,” he told reporters outside the school on Monday.

“We have briefed the staff about the situation, asked those who felt that they had close contact that they might need to self-isolate.”

Mr Whitby said the school would be closed on Tuesday.

• Related: NSW Health known flights with confirmed COVID-19 cases

Willoughby Girls High School is “temporarily” closed from Monday due to a confirmed case of COVID-19, while Epping Boys High School in Sydney re-opened after a one-day closure on Friday because a 16-year-old student was diagnosed with the virus.

The school said on Facebook that the education department and NSW Health have been working to identify all people who had close contact with the year 11 student.

They’re being asked to self-isolate for 14 days.

NSW Health said St Vincent’s Hospital in Sydney is also contacting patients after a man in his 70s visited the emergency department on Friday and was diagnosed with COVID-19.

“The source of his infection is under investigation as he has had no recent overseas travel,” the health department said.

Tweet from @NSWHealth

Victorian cases ‘return travellers’

Authorities confirmed on Monday two of the most recent cases are returned travellers from the United States and Iran.

The most serious case is a woman in her 50s who returned from Tehran via Kuala Lumpar on MH0149, arriving on March 6.

She is in hospital in isolation being treated for pneumonia and is in a stable condition after becoming unwell in Tehran on February 18 with a cough, fever and shortness of breath.

On arrival in Melbourne she was immediately driven to hospital by a relative.

• Related: Smart Traveller 

A man and woman who travelled from the US are recovering in home isolation.

The man in his 50s was on flight UA60 on February 29 and has a very mild illness.

It is not clear if he contracted coronavirus from the flight or while overseas.

Before he was diagnosed he visited Wine by Sam in Seymour on March 3 for a winery tour and lunch, and attended Cinema Nova at Carlton from 7.30pm to 10.30pm for a private screening of The Amber Light.

The second traveller from America is a woman in her 20s who arrived from Colorado on QF94 from Los Angeles to Melbourne on March 6.

Western Australia confirms 5th case

A woman aged in her 60s has contracted coronavirus from her husband after he returned from Iran, making her the state’s first person-to-person transmission of COVID-19.

The woman is WA’s fifth case of coronavirus.

WA Health Minister Roger Cook said her husband had returned from Iran in the past fortnight and was not being considered the state’s sixth case as he was apparently no longer symptomatic.

Australia imposed a travel ban on Iran on February 29.

“Community spread is an inevitability – it’s a question of when not if,” Mr Cook told reporters on Monday

The state government will open COVID-19 clinics within three Perth hospitals this week.

-with AAP