Acting Premier James Merlino confirmed restrictions will ease at 11:59pm on Thursday, but Melburnians must continue wearing masks indoors and outdoors.

"It is inconvenient but an easy thing to do. We are used to it, we know it works," he told reporters on Thursday.

"As we come out of this period of lockdown, while we are driving these new cases to ground, it is a logical and common-sense step to take."

Deputy Chief Health Officer Allen Cheng said masks had proven to be effective at limiting the spread of coronavirus during the state's second wave.

"It's just a small thing we can do to help prevent infection as everything else starts to move again," he said.

While conceding transmission is 20 times less likely to happen outdoors, Professor Cheng said it "doesn't mean it's zero".

"There is still a risk and there have been transmissions documented outdoors," he said.

Prof Cheng said Thursday's four new locally acquired COVID-19 cases were a man in his 80s, a woman in her 70s, a man in his 50s and a man in his 20s, all from the same Reservoir household in Melbourne's north.

One of the family members is on a disability pension and another is a registered carer for them.

The first case in the family, the eldest man, developed COVID-19 symptoms on Monday and was tested the next day, meaning exposure sites were limited to four locations: BP and Bunnings in Thomastown, Coles at Bundoora Square and Marco Fine Foods and Groceries in Reservoir.

Prof Cheng said the family were not close contacts of previous cases and haven't visited any listed exposure sites, although they live close to the City of Whittlesea outbreak.

"These new cases are really the strongest reminder that we are by no means out of the woods yet," he said.

Authorities are also awaiting genomic testing to see if they have the Delta or Kappa strain of the virus.

Their close contacts are self-isolating and some have already returned negative tests.

Meanwhile, state health authorities have started interviewing an infected Melbourne couple who travelled through NSW and Queensland.

A woman left Melbourne with her husband on June 1, when the city was in lockdown, and tested positive at the end of a road trip through NSW and into Queensland.

Her husband has now tested positive as well, although it appears they are late in their infection period.

According to Queensland health authorities, the couple did not apply for a travel exemption to enter the state.

While unable to initially pinpoint a possible source of their infections, Prof Cheng said one of them checked in near the Craigieburn Central shopping centre on May 23.

There have been nine cases linked to the shopping centre.

He said further easing of restrictions next week would depend on the trajectory of the outbreak.

"It's a day-by-day proposition," he said.

"We may need to hold at the current settings for a little longer but we'll have more to say on this as investigations proceed."

Last on Thursday, the health department reported wastewater detections of the virus in the suburban Pascoe Vale, Scoresby and Vermont areas.

Anyone in those areas with symptoms is urged to get tested.

Victoria's COVID-19 commander Jeroen Weimar said 1500 primary close contacts had been cleared in the past 24 hours, as the number of exposure sites fell from more than 350 to below 140.

There are 78 active cases across the state, down five from Wednesday.

Some 23,679 Victorians were tested on Wednesday and 20,784 received a vaccine dose at state-run sites.

Australian Associated Press