Acting Chief Health Officer Ben Cowie on Thursday announced the change, which is effective immediately.

"If you're sitting at seven weeks, eight weeks, nine weeks (between doses), go and get your double dose now," he said, noting more than 52,000 appointments were available at state-run vaccination hubs in the coming weeks.

Professor Cowie said the change, as well as additional supplies of the Pfizer vaccine from the Commonwealth government, will help the state reach its immunisation targets faster.

"But more importantly, they will really contribute to the response to community transmission, put a downward pressure on cases, and contribute to protecting the health system," he said.

The wait time between AstraZeneca doses is now the same as for Pfizer, which was extended from three to six weeks due to limited supplies.

The Australian advisory group on vaccinations recommended a shorter interval of four to eight weeks between AstraZeneca doses in outbreak settings, as COVID-19 cases in NSW began steadily rising in mid-July.

Clinical trials have shown the vaccine is most effective with a dosing interval of 12 weeks.

But Prof Cowie said the state had to strike a balance between long-term vaccine efficacy and getting the highest level of protection against the Delta variant.

"We are recommending six weeks to get that balance between optimal efficacy, and getting as many second doses into people as we possibly can," he said.

Of the state's 176 new cases, 83 are linked to known outbreaks, with the source of the remaining 93 infections under investigation.

Sixty-seven cases are located in the northern suburbs of Melbourne, 61 in the west, 22 in the east and south, 13 in the regional town of Shepparton and one case in Geelong.

"There's not one corner of metropolitan Melbourne that's not touched by this virus," Prof Cowie said.

Health Minister Martin Foley said an outbreak at a Melbourne call centre is of particular concern to authorities, as the office space is shared with Healthcare Australia. It provides workers for the state's vaccination hub.

Seventeen people who work at the call centre have tested positive and 400 primary close contacts are now self-isolating.

A case at Base Backpackers in St Kilda is also a concern. The person has been moved into hotel quarantine.

It is the state's second consecutive day of more than 100 locally-acquired cases, after 120 were reported on Wednesday. It is the highest daily figure since August 22 last year, when 202 infections were recorded.

The government has conceded efforts to bring cases down to zero have failed, with tough restrictions to stay until at least 70 per cent of eligible Victorians are fully vaccinated.

Some reprieve will be granted when 70 per cent have received their first vaccine dose, which is forecast to happen around September 23.

Of the state's 1029 active cases, 61 are in hospital, including 20 in intensive care, 13 of whom are on a ventilator to breathe.

Australian Associated Press