A Victorian teenager has tested positive to the coronavirus while holidaying in NSW after a bungle by health authorities.

NSW chief health officer Kerry Chant said the teenager – who was not from a Melbourne COVID hotspot – went on holiday after Victorian authorities said he had tested negative for the virus.

“Following their arrival in NSW, they were then advised that there had been an error and, in fact, the test result was a positive,” she said on Wednesday.

The teenager and his family are now in isolation in Merimbula, on the NSW south coast. Eighty people who were in the nearby Tathra hotel when the family visited have been contacted and warned.

The bungle, which came as Victoria confirmed 134 new coronavirus cases on Wednesday, will fuel fears Melbourne’s surging outbreak will cross state borders to infect more Australians.

Victoria has 860 active COVID cases, with 41 patients in hospital, including seven in intensive care.

The surge in community transmission of the virus has led to several states closing their borders to Victorians.

Cars banked up at the Albury border crossing on Wednesday.

There was a rush on the NSW border late on Tuesday before it shut indefinitely at midnight, for the first time in a century.

NSW Premier Gladys Berejiklian warned the widespread community transmission of COVID-19 in Victoria was a huge risk for NSW. She urged residents in Albury and other border towns not to travel to other parts of NSW – and said she had been told a possible coronavirus case was being investigated on the Victorian side of the border in Wodonga.

“The probability of contagion in NSW, given what’s happened in Victoria, is extremely high,” she said.

“The probability that we need to be tougher on broader restrictions is extremely high.

“The government is considering what further action we take to make sure the bubble of these border communities is maintained.”

Ms Berejiklian said more than 50,000 exemption permits had already been issued to people living in border communities, allowing them to cross between Victoria and NSW.

“When you have communities mixing with each other, crossing a border, it carries enormous risk,” she said.

“The government is considering what further action we take to make sure the bubble, as we call it, of those border communities, is maintained.”

NSW had eight new COVID cases on Wednesday. Seven were in returned travellers and one in a woman in her 30s from Sydney’s south-west. It is under investigation.

On Wednesday, the Northern Territory has declared all of Victoria a coronavirus hotspot and closed its border with the southern state.

“The territory’s borders will stay closed to all of Victoria until further notice. They are shut indefinitely,” Chief Minister Michael Gunner said.

On Tuesday, Premier Steven Marshall said Melbourne’s continuing rise in COVID cases and the decision to close the NSW-Victoria border vindicated South Australia’s decision to delay reopening all its borders.

Firefighters in PPE prepare to deliver supplies to a Melbourne housing tower. Photo: AAP

Earlier, Victorian Premier Daniel Andrews said 11 of Wednesday’s cases were linked to known outbreaks and 123 were under investigation.

They included 75 linked to the public housing towers that went into a hard lockdown on Saturday.

From midnight Wednesday, five million people in metropolitan Melbourne and the Mitchell Shire, immediately north of the city, will be back in stage-three lockdowns for six weeks.

“I apologise for the inconvenience, the great challenge that many, many Victorian families are going to have to experience over these next six weeks,” Mr Andrews said.

“It’s not where we wanted to be. But we can’t go back. We can’t change those things that have got us to this point. We can only respond appropriately.”