It can't be said there's no risk of coronavirus entering Western Australia if the border is opened, the High Court has been told.
Billionaire miner Clive Palmer's legal challenge against WA's border closure arrived at Australia's highest court on Tuesday, with his lawyers arguing the measure isn't reasonably necessary to stop the reintroduction and spread of the virus.
They argue it breaches the "absolute freedom of intercourse" under section 92 of the constitution.
WA Premier Mark McGowan recently announced the state would reopen its borders to all states and territories from November 14, with some restrictions remaining for NSW and Victoria.
WA's solicitor-general Joshua Thomson said the time frame ensured two incubation periods of the virus - 28 days - would pass after NSW allowed Kiwis to arrive without undergoing quarantine.
Mr Thomson said it was wrong for Mr Palmer to argue the borders should be open because it didn't take into account the nature of the virus.
He noted there was currently no vaccine or cure, it can be asymptomatic, highly infectious and that testing was imperfect.
Mr Thomson also pointed to the risk of people border hopping to get around the rules.
"You can't expect people to act honestly," he said, describing border hopping as a "real, not fanciful risk".
He emphasised the temporary nature of the border closure and health directions, saying they had been tweaked in response to the changing circumstances of the pandemic.
"The only time you get to a situation where the hard border, or the exclusion, is not relevant, is if there is no real risk at all of reimportation (of the virus)."
Mr Palmer's team argues a "hotspot" system is a practical alternative to the border closure, substantially reducing the risk of bringing COVID-19 into the state.
But Mr Thomson said that approach was less effective, as were social distancing and wearing masks.
In June, High Court Chief Justice Susan Kiefel remitted part of the matter to the Federal Court for trial.
Federal Court Justice Darryl Rangiah found the border restriction was the most effective way for WA to deal with the coronavirus.
On Tuesday, Mr Palmer's lawyer Peter Dunning QC pointed to Tasmania, South Australia, ACT and the Northern Territory as being low risk for coronavirus.
He said allowing people in from such areas wouldn't heighten the risk of spread in WA.
State attorneys-general have intervened in the case, arguing the WA directions do not breach section 92 of the constitution.
The federal government withdrew from the case in August.
A state of emergency was declared in WA in mid-March after the state recorded its first coronavirus case in February.
It has been continually extended by the state government.
On April 4, WA police chief Chris Dawson issued directions declaring: "A person must not enter Western Australia unless the person is an exempt traveller."
Mr Palmer - whose company Mineralogy has offices in Brisbane and Perth - sought permission on May 18 to enter the state but was refused.