The billionaire mining magnate is arguing the restrictions are contrary to section 92 of the constitution which provides for freedom of movement between the states.

Evidence is also being given in the Federal Court by the Commonwealth, which opposes WA's hard border closures.

A two-day trial had been listed to start next Monday ahead of the matter returning to the High Court.

But WA's Solicitor-General Joshua Thomson SC sought an adjournment on the basis that the state's expert, Australian National University epidemiologist Kamalini Lokuge, had been called to Melbourne.

The court heard Associate Professor Lokuge had accepted a request from Victoria's Chief Health Officer Brett Sutton last week to help manage the state's public health response to the COVID-19 outbreak, having done similar work during the Ebola crisis in West Africa.

Justice Darryl Rangiah on Thursday accepted that Prof Lokuge was working long hours and had been forced into an "invidious" position of choosing between legal obligations and contributing to potentially life-saving work.

"I'm satisfied that her work is of considerable importance to the Victorian community and the Australian community generally," Justice Rangiah said.

He granted an adjournment until July 27, noting that Prof Lokuge could be taken away from her work in Melbourne for up to three days given she might be required to give evidence as well as contributing to a joint expert report.

A third day for the trial could be required given the slow pace of the proceedings to date, Justice Rangiah added.

Solicitor-General Stephen Donaghue QC has signalled that the Commonwealth will contribute expert evidence that targeted quarantine measures are just as effective as state border closures in managing potential COVID-19 outbreaks.

Justice Rangiah said it was hoped the matter could be listed in the High Court by September.

Australian Associated Press