The most glaring figure in Thursday's update was active cases dropping by 2291, from 7155 to 4864.

It came as Victoria recorded 13 deaths and 240 new cases.

Victorian deputy chief health officer Allen Cheng explained the sharp drop in active cases was due to a process where people left isolation when their symptoms have cleared and enough time has passed since the onset of their illness.

"It's not a simple process," Professor Cheng told reporters.

"A lot of work has gone in over the last couple of days to clear people from isolation so that they can go back into their normal activities."

Victoria's latest deaths take the state's death toll to 376 and the national figure to 463, while 240 new cases extended the state's streak of daily numbers under 300 to five days.

That was the more significant figure to Prof Cheng, still too high for his liking.

Victoria's Department of Health and Human Services are searching for the root of a cluster across a public healthcare network in Melbourne's southeast.

Peninsula Health's outbreak has infected 51 health workers, with 211 close-contact staff furloughed while self-isolating.

Premier Daniel Andrews said an incident management team had been sent to Frankston Hospital, operated by Peninsula Health, to find the source.

"There's no effort being spared to find the root cause as well as proper treatment for those who might be impacted," he told reporters.

Peninsula Health confirmed 40 nurses, four doctors, five support staff and two allied health workers were among the 51 infected staff members.

Another 48 positive patients remain in its care.

"We are still investigating if these cases are the result of increased community transmission, or if it was acquired through the hospital," chief executive Felicity Topp said in a statement.

Mr Andrews wants answers on how the virus spread, but admits authorities may never know.

It comes as active cases among healthcare workers fell from 1065 to 753 - a drop of 312.

Meanwhile, the premier denied he misled the public when he said on August 4 that out of 3000 people who were doorknocked while they should have been self-isolating, 800 could not be found.

At the same time fines were beefed up to $4957 for not isolating, an exercise loophole closed and extra defence force and DHHS personnel added to conduct random checks

But Victoria Police revealed on Wednesday that just 42 fines had been issued from 30,000 doorknocks since April.

"We referred it to Victoria Police. We didn't assume that they were all doing the wrong thing," Mr Andrews said.

Eight of Wednesday's 13 deaths - including a man in his 70s, six women in their 80s and five women and a man in their 90s - are linked to aged care.

A 30 per cent drop in testing continues to worry authorities, with people urged to come forward if their symptoms are mild.

Mr Andrews is stressing the need for solid testing numbers to ensure authorities have a proper picture of Victoria's second wave.

But he was encouraged that the tests for the past 24 hours were back up above 20,000.

Melbourne is in the third week of a strict level-four lockdown, while the rest of Victoria is under level-three. The measures are due to end on September 13.

The premier said it was too early to tell when Melbourne's 8pm-5am curfew would stop.

A ban on evictions and rent increases for residential and commercial tenancies has also been extended until December 31.

Australian Associated Press