NSW has reported 18 more COVID cases, including two linked to a concerning cluster at a Sydney school where the origins of the outbreak remain a mystery.

It comes as a leading infectious disease expert warned the outbreak at the Tangara School for Girls in Sydney’s north-west might be the “tip of the iceberg” for NSW.

The outbreak linked to the independent Catholic school had 17 confirmed virus cases on Tuesday: 11 students, four teachers and two social contacts. By Wednesday that had grown to 19 infections.

“Patient zero” in the growing outbreak remains a mystery.

Elsewhere, Victoria’s coronavirus death toll hit a tragic new high on Wednesday, with 21 deaths confirmed. There were also 410 new COVID infections – a three-day high.

Victoria’s virus toll has risen to 267 – with more than 100 deaths in the past week alone. Australia’s national toll is 352.

In NSW, schools remain a concern, with the director of the National Centre for Immunisation Research and Surveillance, Kristine Macartney, saying the outbreak at the Tangara School could indicate the virus was more widespread in the state than currently thought.

“I think the situation that we’re seeing unfold in term three is indicative of the fact that there is some community transmission now in NSW, which is going undetected,” she told the ABC.

“Schools are probably acting as the tip of the iceberg.”

On Tuesday, NSW Premier Gladys Berejiklian again warned schools about heeding coronavirus restrictions.

“Schools, in particular non-government schools, cannot undertake those extra-curricular activities that you do outside of a pandemic – and I can’t make that message stronger,” she said.

A spokesman for Tangara said the school had held no camps for students since March, when restrictions were imposed.

But a group of students attended a retreat organised by the Eremeran Hills Study Centre and held in Bargo, 100 kilometres south-west of Sydney, without teachers.

On Wednesday, the Pennant Hills centre announced on its website that it was closed until further notice “because we have been informed by NSW Health that individuals who have attended activities organised by Eremeran have tested positive to COVID-19”.

“We can confirm that there was a recent retreat attended by five high school girls in year 10 and 11 organised by Eremeran,” the statement said.

“We are assisting NSW Health in their endeavours to ascertain whether this may have contributed to the outbreak.”

Elsewhere, the NSW Department of Education said Parramatta Public School would be closed on Wednesday for cleaning after a student tested positive to COVID-19.

The school will work with NSW Health to establish close contacts while students learn from home.

It is near Our Lady of Mercy College in Parramatta, where two students tested positive for coronavirus earlier this week. NSW Health said a third case linked to the school had been confirmed on Wednesday.

Because of the growing number of infections, the college will be shut for cleaning until August 24.

On the South Coast, Batemans Bay Public School and Bateman’s Bay High school have re-opened after being closed this week after students tested positive.

“We are in a pandemic – every organisation, every entity needs to abide by the COVID-safe plans,” Ms Berejiklian said on Tuesday.

“My anxiety has not subsided in relation to what a knife-edge NSW is on, but we need to keep pulling together and keep doing the right thing.”

Elsewhere in NSW, a shopping centre at Castle Towers was also closed on Tuesday after NSW Health issued an alert asking anyone who visited it on Friday between 3.30-5pm to monitor for COVID-19 symptoms and get tested if symptoms develop.

Meanwhile, Huskisson restaurant Wildginger has closed after two holidaying patrons who dined there on Saturday night tested positive to the virus.

“Please be advised that at this time, NONE of our staff have tested positive for COVID-19 and the incident was from patrons visiting from Sydney,” restaurant’s management posted on its Facebook page.

“We are devastated at the news of this and will be closed for the next two weeks whilst our affected staff are in mandatory, two-week quarantine.”

Dooleys Lidcombe Catholic Club in Sydney’s west was also forced to close on Tuesday for deep cleaning due to a positive case of COVID-19.

The club is providing NSW Health with contact details of employees and patrons on the premises between 5pm Friday and 1.30am Saturday.

But due to the club’s mandatory use of face masks, NSW Health said the risk to patrons and employees was low. They must still monitor for symptoms until Friday.

The Independent Education Union is urging the state government to mandate mask use for students and staff at senior schools to prevent further outbreaks in schools.

-with AAP