NSW may re-introduce tough new restrictions as the risk of contagion grows.

The New South Wales government has warned it is prepared to enforce tough new restrictions on travel and gatherings as the risk of contagion grows.

People in NSW, particularly those living in border communities, could see tougher restrictions come into place around hotel quarantine, gathering limits and border control.

Health authorities in NSW were yesterday scrambling to contain two major breaches of the tough new border restrictions, after plane passengers from Melbourne left Sydney Airport without screening and a Victorian child tested positive for the virus while holidaying on the South Coast. 

NSW Health confirmed a Melbourne teenager had tested positive for coronavirus while holidaying on the South Coast, after Victorian authorities had mistakenly given him a clean bill of health when he was swabbed for COVID-19 there.

The teenager's family had visited the Tathra Hotel while infectious, prompting the contact tracing and swabbing of more than 80 people who also attended the venue.

A health spokesperson said all passengers on board Tuesday's Jetstar flight were screened in advance of leaving Victoria but 48 of the 137 passengers left before health staff finished screening a prior flight.

"Health staff, airport staff and NSW Police acted quickly to bring passengers back to the gate to be screened," the spokesperson said.

It said 45 of those passengers were contacted and arrangements have been made for them to be screened, while the other three have refused and were referred to the police.

NSW Health said as a result of the breach flights would no longer be allowed to land in NSW until NSW Health teams were in place at airports to screen them.

NSW health authorities are also concerned after a new case of COVID-19 was detected yesterday in Albury and one in Wodonga. Authorities fear that the virus could be brought across the border unless restrictions are tightened.

Mary-Louise McLaws, an epidemiologist and World Health Organization adviser, said there was a "magic number" of active cases in NSW where authorities were so overwhelmed they could no longer do effective contact tracing.

NSW yesterday recorded eight new COVID-19 cases including seven in hotel quarantine and a woman in her 30s from southwest Sydney. The woman's case remains under investigation. 

A teenager on holiday in Merimbula from Victoria has also tested positive for the virus. 

Three new cases of coronavirus were also detected in Canberra furthering concerns the virus could begin circulating in NSW.

There are 454 active cases of COVID-19 in NSW, although the majority of those are overseas travellers in hotel quarantine.

From Wednesday, restrictions tightened furtherand all Victorians are banned from travelling to NSW unless they live in a border community or qualify for an "exceptional circumstances" exemption.

However New South Wales Premier Gladys Berejiklian says more than 125,000 exemption permits have been issued since the state announced it was closing its border to Victorians.