Western Australia's determination to keep its borders closed to all states, not just virus-plagued Victoria, is set to be scrutinised by the nation's highest court.
It comes as WA recorded two new coronavirus cases on Wednesday, both returned overseas travellers who are in hotel quarantine.
There are now 13 active cases in the state. WA's overall tally has increased to 624 after serology tests identified a historical infection in an overseas traveller who returned in April.
Billionaire mining magnate Clive Palmer's legal challenge against WA's border closures is before the Federal Court, with hearings listed for next week.
Once the facts of the case are established, it will be heard in the High Court.
The federal government, which opposes WA's hard interstate border restrictions, is set to give evidence in the matter.
WA Premier Mark McGowan this week wrote to Prime Minister Scott Morrison urging the Commonwealth to withdraw its involvement, saying it made no sense to challenge WA's position while also endorsing the NSW-Victoria border closure.
But Attorney-General Christian Porter says it is appropriate for the Commonwealth to provide its view on such matters, noting that Victoria is the only remaining state with significant community transmission of COVID-19.
"The question is whether or not each state is properly adapting its rules and its border restrictions to the problems it's facing," Mr Porter told 6PR radio.
"It's a fluid situation and it's a matter of assessing evidence from experts before the courts.
"The constitution has provided for the establishment of a nation and part of the core feature of a nation is the ability to move across state borders."
Mr Porter said it defied logic to claim that it could be unconstitutional to open the WA border to some states but not others.
He rejected calls for Canberra to stay out of the dispute, saying the idea the Commonwealth would not be heard in a constitutional matter was "ridiculous".
"We're not assisting Clive Palmer, we're assisting the court," he said.
Mr Palmer, a noted critic of the WA premier, has accused the McGowan government of discrimination against Queenslanders.
"We haven't had any COVID-19 cases for weeks now, yet the WA Premier keeps Queenslanders out of his state," he said.
Prime Minister Scott Morrison on Wednesday said his position on border closures beyond Victoria had not changed.
"That outbreak is being contained now at the Victorian border," he said in Canberra.
"Arbitrary decisions about state borders is a separate issue and we'll continue to maintain our position that Australia is one country."
Federal Labor frontbencher Chris Bowen backed WA's calls for the federal government to withdraw its intervention in the Federal Court matter.