Sydney's booming Covid-19 cluster has crossed state borders with two Victorians forced to isolate after testing positive.

Victoria's Covid Commander Jeroen Weimar said three of the family of four from the City of Hume in Melbourne arrived on a flight from Sydney on July 4 carrying red zone permits.

The other person entered Victoria by road on July 8.

They all initially tested negative after arrival but two became symptomatic and were tested on Sunday with a positive result recorded on Monday morning.

Medical officer Ahmed poses for a photo at the Fairfield Showgrounds Covid-19 testing clinic in Sydney. (Photo / Getty Images)

Weimar said the delayed onset of the virus showed the "importance of the red zone permit system".

The flight has not been listed as an exposure site as the three family members tested negative two days later and all other passengers remain in isolation as part of Victoria's red zone restrictions.

Adding to the tension is the positive test of two members of a removalist crew who travelled through Victoria and SA.

The crew made stops at two Victorian family homes on July 8, one in the City of Whittlesea and the other in Maribyrnong.

The City of Whittlesea family had recently relocated from the interstate and tested negative on July 9.

The family of four from Maribyrnong who came into contact with the removalists remain in Victoria in temporary accommodation, have been isolating and so far none of them has tested positive to Covid-19.

The cases come after Victoria recorded its 12th day of no locally acquired cases and the state effectively shut the border to NSW and the ACT overnight, declaring them red zones under the travel permit system.

Sydney's lockdown could last until the end of August, one expert says, pointing to the difficulty Melbourne experienced getting its cases down during its second lockdown.

On Monday, a record 112 cases was announced in Greater Sydney with NSW Premier Gladys Berejiklian acknowledging it would be "almost impossible" for authorities to lift the lockdown on Friday as scheduled.

While the state has not yet revealed how long any extension would be, some experts have suggested it could last between three to four weeks.

Restrictions in Sydney are not as strict as those in Melbourne during its second lockdown. (Photo / NCA Newswire)

Professor Michael Toole, a leading epidemiologist at the Burnet Institute, pointed out Melbourne took six weeks to get its Covid cases down to almost zero from around 100 during its second lockdown last year.

The city — which endured a three and a half month lockdown last year — recorded 100 cases on August 30, 2020.

"It reached almost zero on October 14, so it took six to seven weeks, and that was with the strictest possible restrictions," Toole told news.com.au.

Greater Sydney's current restrictions are not yet as strict as Melbourne's Stage 4 rules, which included a curfew, 5km travel limit and the closure of non-essential retail shops.

"I went back and looked at the Burnet modelling after the second wave to try and identify what measures led to a significant change in the outbreak and the rate of new cases reported," Toole said.

Covid-19 test numbers have risen in Adelaide. (Photo / News Corp)

"When Stage 3 restrictions were introduced, 14 days after there was a distinct change, it almost flattened the curve but it wasn't enough to force a downward trend.

"Masks probably had the biggest impact but it still wasn't enough so cases kept growing.

"Stage 4 basically led to the end of the outbreak but it took some time."

Toole believes Sydney should make its current lockdown more strict.

"I think we've shown over time that the faster and harder you go in, the shorter the lockdown is," he said.

"I think Sydney's just got to buckle down."

- by Mitchell Van Homrigh, news.com.au