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Experts call Melbourne COVID-19 spike 'unique' as they tell what NSW can do to stop own second wave

Sydney can avoid the city-wide shutdown re-imposed on greater Melbourne with localised lockdowns to prevent a second wave of COVID-19 cases.

Sydneysiders have so far avoided the explosion in cases experienced in Victoria- despite 21 cases now being linked to an outbreak of the virus at a pub in Casula in the city's south-west.

But University of New South Wales epidemiologist Professor Mary-Louise McLaws said the spike in NSW was very different to how Melbourne's outbreak spread.

'What happened in Melbourne was incredibly unique,' she said.

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Police question a pedestrian in the Melbourne CBD on Monday as the city endures a six-week COVID-19 lockdown. A leading epidemiologist has said the cirumstances around the city's outbreak were 'incredibly unique'

'We had highly interconnected family clustering. And that clustering was very big, with many people living in high density with close social and family connections.' 

She told ABC News health authorities still had to act quickly with the outbreak at the Crossroads Hotel - adding the virus could have spread to anywhere in the state from the popular truck stop.

Australian National University infectious diseases expert Peter Collignon said the pub outbreak in Sydney's western fringe was concerning.

He added health authorities could stem the outbreak with rigorous 'ring-fencing' in infected suburbs.

Such a strategy would mean certain areas of the city may be put into lockdown without having to impose stage three restrictions on the entirety of greater Sydney.

'If you have an outbreak in Bondi, I am not sure Penrith necessarily has to be shut down,' he said.

People queue up in their vehicles for a COVID-19 test at a testing station at the Crossroads Hotel in south-western Sydney - linked to at least 21 infections of the virus - on Monday. Another expert said localised lockdowns in hotspot suburbs could stem the spread of the virus in New South Wales

Professor McLaws had earlier said the point at which authorities could no longer effectively use contact tracing to suppress an outbreak was 100 cases over the course of two incubation periods.

COVID-19's incubation period - the time between exposure to the virus and the first appearance of symptoms - is between one and 14 days, according to the federal government's Department of Health.

The Casula outbreak has been linked to five other venues in Sydney, including a local gym, Canterbury Leagues Club and Sydney's famous Star City Casino.

A health worker in protective equipment administering COVID-19 tests to people in their cars at the Crossroads Hotel testing centre in Sydney on July 13

Growing concern about the spike in cases has led South Australia Premier Stephen Marshall to say he will be extensively reviewing his state's scheduled reopening to NSW and SA.  

SA is due to lift restrictions from July 20 but Premier Steven Marshall says there's a question mark over the existing timetable.

'We've got to have a very close look at what's happening with that cluster [Casula outbreak], that's raising some real queries,' he said.

Pictured: The Sydney venues feared to be at the centre of COVID-19 outbreaks

'We've just got to see if there is a significant escalation between now and the 20th of July.

'But if it's not safe to lift our border restrictions then we will not be doing so.'

SA has previously lifted the quarantine restrictions for people coming from Queensland, Tasmania, the Northern Territory and Western Australia.

Australian National University Professor Peter Collignon

But it has imposed a hard border closure with Victoria, only allowing locals to return and essential travellers through, because of the surge of infections in Melbourne.   

Victorian authorities are meanwhile scrambling to suppress a second wave of the coronavirus as new outbreaks pop up across Melbourne.

Of 177 new COVID-19 cases reported on Monday, 151 remain under investigation, a trend that has become a constant in the last week as health authorities work hard to establish links between cases.

Victorian Chief Health Officer Brett Sutton said the latest infection tally followed three consecutive days of case numbers exceeding 200.

'I'm not going to be complacent about today's number, it is great that it is lower than our peak, but we may not have reached our peak yet,' he told reporters.

'I would like to see a week of decreasing numbers before I say I have greater confidence about the direction we're going in.'

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