Police have detained two teenage girls missing from Victoria who hitch-hiked into South Australia and then boarded a school bus, triggering a coronavirus scare.

Police said the girls used several vehicles to travel into the state to see a third girl in Goolwa – more than 200 kilometres from the Victorian border.

The trio then travelled together on a school bus from Goolwa with other students to Victor Harbor High School, south of Adelaide, on Friday morning.

It came as the coronavirus outbreak that has sent Victoria into a seven-day lockdown grew to 30 more infections on Friday. Health authorities confirmed four more cases, including the first in the outbreak outside close contacts of those already infected.

Elsewhere, NSW Health issued an urgent warning for tens of thousands of Sydney residents, after fragments of COVID-19 were found in a sewerage network.

The Homebush network site encompasses about 40,500 people living in Homebush West, Strathfield, Concord West, Sydney Olympic Park, Wentworth Point, Newington, Lidcombe, Homebush, Silverwater, Petersham, Liberty Grove, Rhodes, North Strathfield and Concord.

In a statement, NSW said it was aware of a recent COVID case who had left hotel quarantine and was living in the area, but urged everyone in the area to be “vigilant” in monitoring their symptoms.

Nearly 40 people were reportedly held on the bus for hours after the girls were discovered. Photo: ABC

Back in South Australia, 37 people aboard the bus with the two Victorian schoolgirls were sent into isolation. SA has closed its border to all travel from Victoria in response to the community cluster that emerged in Melbourne’s northern suburbs this week.

SA Police described Friday’s situation as a “COVID incident” and said everyone on-board the bus would need to be tested and isolate.

Incident controller Hans Minchin said the two teenagers were being held under police guard at the Victor Harbor Police Station.

“There were 36 in there in total, with the bus driver [in addition] as well,” he told the ABC.

“That was with the two teenagers. They’ve been taken off for testing.

“They’ve been taken to the hospital, the COVID clinic at the hospital, but they’re not there now. They’ve had their tests done and they’re under police guard at the moment.”

SA Police has also directed anyone who might have given the girls a lift on their trans-border journey to “isolate immediately” and contact the SA Health COVID-19 hotline.

Victor Harbor High School deputy principal Adie Conley confirmed the incident in a statement posted to Facebook on Friday.

“This morning SAPOL stopped a school bus with some of our students on it in relation to a possible breach of COVID-19 restrictions,” the message stated.

“All families of children on the bus have been contacted and their children are in the process of being taken home.”

One parent, Skye, told ABC Radio Adelaide that her son was stranded on the bus for about three hours, and police had given those on board masks and secured the area with tape.

She said the two teenagers told the driver they were new students.

“They didn’t have backpacks, but they were in uniform,” she said.

“They said they didn’t have their bus passes … [but] they had apparently told the bus driver they were new students. None of the kids were any the wiser.

“Once they arrived at the school, the police were already there. I believe that the girls’ parents had contacted Victorian police, and then the Victorian police contacted SA Police.

“I’m quite angry about it. I just think it’s really selfish for them [the hitchhikers] to put so many families in such a predicament.”

It is unclear where the girls originally came from, and whether they travelled from Greater Melbourne or other coronavirus-affected parts of Victoria.

Vic fears focus on city bars, pubs

Earlier, Victorian authorities issued an urgent alert for five Melbourne bars after a confirmed COVID case in a pub patron.

It is the first community transmission of the virus outside the cluster that started in Melbourne’s north earlier this week.

The patron, who was one of four new Victorian infections on Friday, is thought to have picked up the virus at the Sporting Globe Bar and Grill in suburban Mordialloc last Sunday (May 23).

“We now see the significant risk, in particular, at those social venues, the bars and clubs … and we are very keen to ensure that people who have been to those places at those times come forward very quickly,” testing commander Jeroen Weimar said on Friday.

The diagnosis has prompted health authorities to name the bar and four other venues as of particular concern, as they try to quell the burgeoning COVID cluster. They are:

• Three Monkeys, 210 Chapel Street, Prahran, May 22
• The Somewhere Bar, 181 Chapel Street, Prahran, May 22
• Palace Hotel, 505-507 City Road, South Melbourne, May 21
• The Local, 22-24 Bay Street, Port Melbourne, May 21
• See a full and updated list of sites and times of concern here

Authorities believe some of the venues had more patrons during exposure periods than QR check-ins have revealed.

“The anecdotal evidence we have is that there were in some of those locations tens or hundreds of people – those are not the numbers we are seeing on the QR systems,” Mr Weimar said.

The state chief health officer Brett Sutton said authorities remained worried about community transmission, particularly in high-risk sites such as bars and pubs.

He said it was still early in the battle against the latest cluster – with 15,000 primary and secondary contacts in isolation.

“Of course four [infections] is better than 10. It is not over yet. We could get 10 tomorrow, 20 tomorrow,” he said.

“We absolutely have to recognise that there are thousands and thousands of close contacts still to go through the incubation period, who could become symptomatic.”

Victoria’s lockdown is scheduled to last until midnight on June 3.

-with agencies