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South Australia has recorded its first case of coronavirus in 17 days after a woman who underwent hotel quarantine in Melbourne tested positive in Adelaide.

The woman returned from overseas to Victoria and was quarantined for two weeks.

She tested negative twice before she was released and allowed to fly home to Adelaide.

But upon arriving home the woman then tested positive.

South Australia has recorded its first case of coronavirus in 17 days after a woman who did hotel quarantine in Melbourne tested positive in Adelaide (pictured, people wearing masks in Melbourne on Thursday)

The woman returned from overseas to Victoria and was quarantined for two weeks (pictured, people wearing masks in Melbourne on Thursday)

A medical worker (centre left) speaks to people queueing outside a COVID-19 coronavirus testing venue at The Royal Melbourne Hospital

Experts say she had a very low level of virus in her system and was not infectious on her domestic flight.  

The woman's close contacts have been tested and returned negative results.

Premier Steven Marshall announced the news on Thursday afternoon.

'We have one new case to report today… she undertook her two weeks of her mandatory supervised quarantine in Victoria where she returned two negative tests,' he said.

'It is a low positive, she is not contagious.'

Deputy chief public health officer Dr Michael Cusack said the woman was probably not contagious on the flight. 

'The remainder of her family have been tested and tested negative.'

'I think we can say with confidence that she was not infectious on her flight'.

Victoria recorded 317 new cases of coronavirus on Thursday - its worst ever daily figure - as the state's second wave continues to grow.  

Two men in their 80s died overnight, taking the state's death toll to 29 and the national death toll to 113.

There are 109 people in hospital in Victoria, with 29 in intensive care.

Just 28 of the new cases announced on Thursday were connected to known outbreaks, while the sources of the other 289 were unknown - suggesting widespread community transmission.

Melburnians exercise along Elwood Beach on Tuesday (pictured) as the city suffers a six-week lockdown due to a spike in infections

A man and woman wearing a facemask walking along Lygon Street in Melbourne on Wednesday (pictured) as Victoria endured an 11th straight day of triple-digit infections

The state has 160 outbreaks ongoing, including in offices, aged care homes and schools.

There are 157 cases now linked to Al-Taqwa College and six cases linked to HWL Ebsworth lawyers in Melbourne.

Premier Daniel Andrews was not yet prepared to move to draconian Stage-4 lockdowns but said Melburnians must be vigilant if that was to be prevented.

'It's well too early for us to be moving to a whole new stage,' he said. 

'If you want to make that less likely, if you want to get out of these sort of restrictions as fast as as possible, then we all have to play our part.' 

Victoria Police issued $107,000 in fines in 24 hours leading up to Thursday's announcement, many to people who were at illegal parties. Gatherings of more than two are banned in Melbourne. 

Chief Health Officer Brett Sutton said the numbers may get worse.

'We not have hit our peak,' he said.

But he was hopeful that the figures will come down once the lockdown kicks in.

'I expect it to turn around this week. But as I always said, it's no guarantee,' he said. 

Meanwhile, New South Wales premier Gladys Berejiklian has vowed to avoid locking down her state despite fears Victoria's outbreak is spreading north of the border. 

'We can't keep going into lockdown and that is something we need to avoid at all costs,' she told KIIS radio.

Victoria has suffered a triple-digit increase in cases for the past ten days.

Asked if Sydney would be locked down if it saw the same infection numbers, Ms Berejiklian said: 'We would avoid that happening.' 

With around one million Australians on the dole, the premier is desperate to avoid the economic damage that further lockdowns would bring. 

She also ruled out suburb-by-suburb lockdown as tried in 12 postcodes in north-west Melbourne.

Sydney residents have been warned to watch for symptoms if they have visited one of these places, which were attended by people who have tested positive

Ms Berejiklian said locking down a suburb where an outbreak happens is ineffective because people who may have been infected there travel 'all over Sydney'.

'It's all or nothing. You can't lockdown a suburb, because [people who have passed through] live elsewhere,' she said.

Health officials are trying to control an outbreak linked to the Crossroads Hotel in Casula, south-west Sydney.

The outbreak is understood to have began when an infected freight driver, known as Patient Zero, went to a work party at the pub after driving from Melbourne.

But Ms Berejiklian said her concern was that community transmission was happening before the border shut down, without being picked up on.

Melbourne is suffering 160 separate outbreaks. Pictured: Map showing some of the outbreaks on Wednesday

'Our case numbers are relatively low but what we don't know is if there was community spread months ago when people from Sydney and Melbourne were moving in and out.

'Things can get out of control very quickly,' she warned.

The premier said Victorian officials failed to spot community transmission was happening before it was too late.

'They didn't pick it up early enough. People who should have stayed at home didn't know they had to stay at home or didn't stay at home and then that spreads through the community

'We're hoping that hasn't happened here in New South Wales.'

An empty Flinders Street Station is seen on Thursday (pictured) as Melbourne endures its second coronavirus lockdown

NSW Health reported 10 new COVID-19 cases in the 24 hours to 8pm on Wednesday from almost 23,000 tests, with six cases locally acquired.

Three-of-the-six cases - a child of a known case, a man in his 30s who was a contact of a known case and a male patron in his 80s - are connected to the Crossroads Hotel in Casula, which an infected Melbourne man attended on July 3.

At least 21 of the 37 known cases in the cluster were hotel patrons.

NSW Chief Health Officer Kerry Chant said two other cases had arisen since 8pm on Wednesday, including a teen who attended the Hurricane Grill in Brighton-Le-Sands on July 11 with a known case. The site has been closed for cleaning.

Another case involved a NSW man who returned to Ballina from Melbourne on Jetstar flight JQ466, with all passengers on that flight being contact traced.

'Obviously we are concerned when we find cases that can't be linked back because it does indicate we have missed a chain,' Dr Chant told reporters on Thursday.

Dr Chant this week warned COVID-19 clusters would pop up because of the 'stealthy' nature of the virus and the rapidity with which the virus could spread.

NSW Health on Wednesday confirmed the 'patient zero' of the outbreak was most likely a Melbourne freight company employee who attended the Crossroads Hotel on July 3 for a work party. Six of his colleagues have since caught COVID-19.

The man entered NSW on June 30, before the state's border with Victoria closed.

NSW Health Minister Brad Hazzard warned it was unrealistic to expect the state to frequently record zero COVID-19 cases as the virus could bubble away under the surface.

Mr Hazzard told 2GB radio on Thursday that young Australians in particular needed to keep in mind the consequences of their actions amid the pandemic.

Medical workers are seen giving coronavirus tests in Sydney's Bondi on Tuesday (pictured) amid fears the Melbourne outbreak may spread to NSW

'If they happen to have the virus they can be super-spreaders with great skill and acumen,' he said.

'We've got a young bloke from that latest cluster ... he had one of the highest infection levels that we've ever seen.'

NSW Health on Wednesday also named several venues where confirmed COVID-19 cases spent time including the YMCA at Revesby, Wests Leagues Club at Leumeah, Macarthur Tavern in Campbelltown and Casula Kmart.

The Milky Lane burger joint in Parramatta and the Bavarian Macarthur restaurant in Campbelltown were later added to the list as well as Woolworths in Bowral.