Mr Thomson said replacing the hard border with a ban on travel from coronavirus hotspot areas created a risk due to the five to seven-day lag before an undetected outbreak was typically identified.

“If you reduce the risk of people with the virus coming in, that means you can relax the measures inside the border for the reason that the risk is nowhere near as great because of the efficacy of the border closure,” he said.

“On the other hand, if you removed the hard border closure then there might be a question about then do you have to reimpose a more stringent form of common measures.”

Common measures include quarantine, self-isolation, testing, movement restrictions, gathering limits, contact tracing and personal hygiene.

Judge Darryl Rangiah, who is presiding over the Federal Court proceedings, questioned whether WA had a 'hard border' due to an average of nearly 3300 people entering the state from the east each week since April 5.

Mr Thomson agreed a better term was to refer to it as WA’s "border strategy".

WA remains the only state with its border 'closed' to every state, with five other jurisdictions which have eliminated community spread of the virus moving to allow travel with other COVID-19-free states while restricting entry from hotspots, or requiring those arriving from hotspots to complete 14 days quarantine.

Mr Palmer’s lawyer, Peter Dunning QC, said WA had, through current measures, shown it was willing to accept a very low level of risk of coronavirus re-emerging in the community.

He said evidence given from each of the five public health experts during the hearing, including WA’s Chief Health Officer Andrew Robertson, estimated the risk of the virus returning to WA from interstate was less than 1 per cent and would remain that way if the border was lifted to all but coronavirus hotspots.

“Once you understand the risk [Dr Robertson] is willing to accept, then questions of whether one [strategy to contain coronavirus] is better than the other are of diminishing utility,” Mr Dunning said.

“The proper inquiry will ultimately become, amongst the available alternatives, is there an equally acceptable one that does not involve such a restriction on interstate intercourse?

“Your Honour should not find that WA is not willing to take the risk of people from eliminated states coming into the state. They don’t want to.

“Their own Chief Health Officer’s evidence is that between the elimination states it’s the same risk as the hard border risk, so that’s the actual risk, rather than the political choice.”

Premier Mark McGowan has said WA's hard border would come down when the public health advice said it was safe to do so.

Under Australia’s constitution, trade and movement between the states “shall be absolutely free”, however movement can be restricted if reasonable in the event of a public health emergency.

Prime Minister Scott Morrison said WA’s “all-or-nothing” approach to its border closure was likely unconstitutional given there was little public health justification.

But the hard border is strongly supported by the majority of West Australians, with a Facebook news poll by Nine News Perth finding 96 per cent of the 132,000-plus respondents supported the strategy.

If unsuccessful in its legal fight, the WA government would have the power to enforce mandatory quarantine and testing of interstate travellers on arrival.

Mr Thomson said other measures, such as people being required to have a COVID-19 test in their state of origin before flying to WA, or wearing a mask while on the plane, could also be considered.

A decision on Mr Palmer's legal challenge, which is supported by the Commonwealth government, is expected to be made in the High Court from September.