Melbourne is going into a 7-day circuit-breaker lockdown from midnight tonight.

Acting Victoria Premier James Merlino said this afternoon the state would go into lockdown from midnight tonight until 11.59pm on June 3, with only five reasons to leave the house.

Those five reasons include to get food and supplies, authorised work, care and caregiving, exercise for up to two hours each day with one other person, and getting vaccinated.

As part of the restrictions:

• People can only leave home for emergency reasons, to get food or to get vaccinated. People must get vaccinated as soon as they are elligble
• Any shopping and excercise must be done within 5km of home
• Masks must be worn when outside the house
• No public gatherings can be held
• Restaurants, pubs and cafes may operate but with takeaways only. Other retail stores can provide click and collect services.
• Schools move to remote learning except for emergency workers' children.
• Hotels, clubs, casinos, community venues and pools are closed.
• Aged-care facilities may have no visitors, and visitors in hospitals are only for births or end of life.
• Weddings cannot proceed unless end of life matters apply and only with limited numbers.

Merlino said the variant was very contagious and there were more than 10,000 primary or secondary contacts of cases in the cluster.

"Our primary concern is just how fast this variant is moving," he said.

"The time between catching virus and passing it on is faster than ever.

"Unless something happens this will become increasingly uncontrollable.

"If we make the wrong choice now, if we wait too long, this thing will get away from us," Merlino said.

"With 10,000 primary and secondary contacts, we have to act now."

Vaccinations at state sites are being expanded to people aged over 40 from tomorrow.

"If you are elligble get vaccinated," Merlino said adding that Victoria was in this position as a result of the hotel breach in South Australia.

"Only way to get through this is for everyone getting vaccinated as soon as they are elligible."

The state's health authorities spent the night debating whether to impose a lockdown of up to seven days. It is the fourth time Melbourne has been placed in a lockdown.

Victoria's Covid-19 outbreak has risen significantly overnight, with the state's cluster now at 26 cases.

One of the 12 cases confirmed by the Department of Health was reported on Tuesday, meaning there were 11 new infections recorded overnight.

One of those is in intensive care on a ventilator.

More than 40,000 test results were received overnight, with the state's active cases now sitting at 34.

On Tuesday Covid-19 Response Minister Chris Hipkins announced a pause on New Zealand's travel bubble with Victoria effective from 7.59pm NZ time.

The pause is effective for 72 hours initially but would be under constant review.

Closing New Zealand's travel bubble with Victoria was the "right thing to do", Professor Michael Plank says, who adds that it should remain shut for as long as needed.

Government officials in met late into Wednesday night to debate the state's next move, with reports further meetings are being held this morning to nail down details of the lockdown.

There are now 79 "exposure sites" visited by the cases - including a McDonald's, Anytime Fitness gym and a Kmart. are among a long list of new venues added to a list of exposure sites, which has now risen to 79.

Of particular concern to authorities is a bar crawl an infectious person went on in South Yarra and Prahran on Saturday night until the early hours of Sunday morning.

As contact tracers do their best to trace everybody who has come into contact with the confirmed cases, exactly how the outbreak began is still a mystery.

Meanwhile other Australian states are introducing restrictions on inter-state travellers in response to the cluster in Melbourne.

For a list of restrictions, click here for Queensland, Tasmania, New South Wales, South Australia, Northern Territory, Western Australia, and ACT.

text by Ben Graham, news.com.au