Western Australia will reintroduce a hard border to Victoria, effectively blocking residents of the entire east coast from travelling west.
From midnight on Friday, WA will reclassify Victoria as a "medium risk" jurisdiction under its controlled border regime.
The change denies entry to non-exempt travellers and applies to anyone who has been in Victoria since the start of its coronavirus exposure period on July 8.
West Australians are urged to return home immediately and will be required to undergo 14 days quarantine upon arrival.
WA already has hard borders in place for NSW and Queensland.
Victoria recorded six new cases on the first day of its snap lockdown. It is dealing with 126 exposure sites, including the MCG, with 1500 primary close contacts and 5000 secondary contacts.
"I want to take this opportunity to urge all Western Australians who want to get home to return without delay," Police Commissioner Chris Dawson told reporters on Friday.
"(People who) delay making plans to return need to understand it may be even more difficult for you to get approval to come into Western Australia."
A 14-day quarantine requirement had already been introduced on Wednesday with immediate effect, catching some passengers aboard flights from Melbourne unaware.
A further six flights were scheduled to arrive on Friday.
WA Chief Health Officer Andy Robertson said Victoria had made the right call in implementing its fifth lockdown since the start of the pandemic.
"Victoria has locked down very quickly so we would hope that would be highly effective and they'll get on top of it very quickly," he told reporters.
"With Sydney ... it almost certainly will take longer. I imagine it would take a couple of months at least."
An outbreak in Perth's northern suburbs has officially been declared over after all cases completed quarantine and produced negative tests.
The cluster, which plunged Perth into a four-day lockdown, was linked to a woman who had contracted the Delta variant while holidaying in Sydney.
About 5200 people are in home quarantine in WA after recently arriving from NSW, Queensland and Victoria.
Mr Dawson said most had done the right thing, despite several people being fined this week for breaching directions.
"We will and do check physically to make sure people are home," he said.
"That's how we've been uncovering those few people that aren't complying with directions, and they run the risk of being arrested."
About 4200 people had arrived in WA from Victoria in the five days prior to the tightening of the border.
Thursday night's AFL game between Geelong and Fremantle at Optus Stadium went ahead with no crowd restrictions.
Mr Dawson defended the decision to let AFL teams enter while other people were being locked out.
"We have had thousands of people who have made applications to come and they haven't all been valid ... we've got to balance up what's in the public interest," he said.
WA this week further tightened exemptions for travellers from NSW, including those granted on compassionate grounds.
The latest travel advice states that WA residents wanting to enter from NSW "are not guaranteed right of entry" and must demonstrate extenuating circumstances.
WA recorded no new cases on Friday.