Victoria will enter a "short, sharp circuit-breaker" lockdown for five days amid fears the highly infectious UK strain of coronavirus has spread in the community.
Premier Daniel Andrews says the state will enter the lockdown at 11.59pm on Friday, with people only able to leave home to shop for food and essential items, provide or receive care, exercise and to work or study if they can't from home.
The rules are largely in line with stage four restrictions imposed last year, with a five-kilometre travel limit, compulsory face masks indoors and outdoors and no visitors allowed at homes.
All non-essential retail will close but supermarkets, bottle shops and pharmacies will remain open. Cafes and restaurants will only be able to offer take-away.
Weddings will not be permitted unless under exceptional circumstances, while funerals will be restricted to 10 mourners.
Gyms, pools, community centres, entertainment venues and libraries will close, though the Australian Open tennis tournament and other sporting events will continue without crowds.
Schools will also be closed from Monday except for vulnerable students or children of essential workers.
The premier says a list of essential workers will be shared on the Department of Health website.
He confirmed people would be fined if they breach the restrictions.
The lockdown is in response to a COVID-19 outbreak at Melbourne Airport's Holiday Inn, which has grown to 13 cases.
Five new cases were recorded on Friday - an assistant manager at the quarantine hotel and four close contacts of people who earlier tested positive.
All five self-isolating at the time they tested positive.
Genomic testing on earlier cases confirmed they have the more virulent UK strain of the virus.
The "working assumption" is all people linked to the cluster have the B117 strain, which Mr Andrews described as "hyper-infectious".
"It is the advice to me that we must assume that there are further cases in the community than we have positive results for," he told reporters.
"It is moving at a velocity that has not been seen anywhere in our country over the course of these last 12 months."
The premier said he was confident the lockdown, similar to what was imposed in Adelaide, Brisbane and Perth after cases escaped quarantine, will be effective.
"We will be able to smother this. We will be able to prevent it getting away from us."
Victoria's Chief Health Officer Brett Sutton said the alternative to the lockdown was "potentially devastating".
"I do not want to be here come Wednesday not having done this and talking about 10, 15, 20, 30 new cases a day, including mystery cases, or cases that we can't chase down," he said.
Victoria's COVID-19 testing commander Jeroen Weimar said more than 900 people were self-isolating after coming into contact with cases.
Authorities are working to track down 5000 people who passed through Melbourne Airport's Terminal 4 after a case worked at Brunetti cafe while infectious on February 9 between 4.45am and 1.15pm.
"We believe at that point they may well have been infectious," Mr Weimar said.
About 29 flights arrived and departed through the terminal during the eight-hour window.
Anyone who visited the terminal then needs to get a test and isolate for 14 days, including those who have since travelled interstate.
"It's critical that we run this to ground now, that we stop it here," Mr Weimar said.
The outbreak can be traced back to a family of three who quarantined at the Holiday Inn and are believed to have been infected overseas.
One family member, who is now in intensive care, used a medical device called a nebuliser in their room, despite them banned outside of medi-hotels.
"Those machines are not allowed. That was clearly communicated. But if you're really inviting me to have a crack at a bloke who is on a machine to breathe at the moment in an ICU, I'm just not doing that," Mr Andrews said.
He flagged changes to the state's hotel quarantine system, including a cap on returned travellers.
On Friday evening, the state government announced a pause on all international passenger flights from Saturday, excluding those already in transit.
"We know this will be difficult news for people who are overseas and want to get home but our focus right now has to be taking this short, sharp action in response to this outbreak," a spokesperson said.
Victorian Opposition leader Michael O'Brien criticised the Andrews government for locking down the state for a third time.
"This government stuffed up hotel quarantine again, this government mismanaged contact tracing again and this government let the virus into our community again," he said.
In response to the outbreak, Tasmania, Northern Territory, Queensland, Western Australia and South Australia have closed their borders to Victorian travellers, while the NSW border remains open.
Meanwhile, the federal government has announced it will lift mutual obligation requirements for job seekers from Friday until Monday, February 22.