The government has revised Victoria's status to one of medium risk, effectively barring travel to WA.

Only people granted exemptions will be allowed to enter, while West Australians returning home will be required to quarantine for 14 days.

Any Victorians who arrived in WA on or after December 21 will also need to self-isolate for 14 days from when they first entered the state.

That will apply to about 16,000 people, although some may already have left after Christmas celebrations.

Those still in WA will have to take a coronavirus test on day 11 or at any time if they develop symptoms.

The same rules are already in place in regard to NSW.

Acting Premier and Health Minister Roger Cook said the new measures were based on the latest health advice.

"Clearly we are dealing with an unpredictable situation that is likely to get worse before it gets better," Mr Cook said on Thursday.

"The timing is terrible but COVID-19 doesn't respect dates.

"This is the best means we have at the moment to protect Western Australians.

"Which is the reason why we've acted quickly and we've acted decisively."

The border closure comes after Victoria revealed its cluster of virus cases had grown to eight, prompting it to close its border to NSW.

SA has also closed its border to NSW but is still monitoring the situation in Victoria.

Police Commissioner Chris Burgess said the government was aware the changed rules would have a significant impact on people currently on flights or about to board flights to WA.

He said they would be greeted by police and directed to isolate for 14 days, or would have the option of returning immediately to Victoria.

"We appreciate that many of those travellers will be surprised that the rules have changed while they're in the air," Mr Burgess said.

"But that is the very nature of this quick changing virus, this pandemic."

Mr Cook said the government recognised the arrangements were not perfect and people would be disrupted.

But he said the government had little choice but to move swiftly.

"It's based on the advice from the chief health officer. In this case, you've seen a situation where there's the outbreak but also the potential for additional cases," Mr Cook said.

"So we're acting out of an abundance of caution and with the sole intent of keeping Western Australians safe."

WA reported two new COVID-19 cases but both were returned travellers in quarantine.

They bring the total number of cases since the start of the pandemic to 861.

Australian Associated Press