Treasurer Ben Wyatt will on Thursday deliver a budget expected to include a modest surplus and massive infrastructure spending.

But there is unlikely to be any clarity on when the state's hard borders will come down.

Tuesday's federal budget included the assumption that WA's borders will remain closed until April 1, after the March state election.

"That's an assumption ... there isn't any science behind that," Premier Mark McGowan said on Wednesday.

"We'll just continue to take health advice."

Health Minister Roger Cook later said the WA budget forecasts would be based on cautious assumptions about travel restrictions.

"It obviously anticipates there will be greater movement over the coming 12 months but it doesn't anticipate that our borders would be reopening anytime soon," he said.

Having gone almost six months without any community spread of COVID-19, WA has refused to set a date for welcoming back interstate travellers.

Under current health advice, the borders won't reopen until the eastern states go 28 days without community spread.

Victoria has continued to record "mystery" cases and NSW on Wednesday identified three new locally-transmitted infections, ending a 12-day streak without them.

In recent weeks, SA has opened its border to NSW, Queensland has promised to follow suit and the Northern Territory has outlined plans to allow visitors from regional Victoria.

Authorities have warned that because WA has largely removed restrictions on physical distancing and large gatherings, it would only take one case coming in from the eastern states to generate a large uncontrolled outbreak.

WA recorded one new case on Wednesday. A man in his 40s who is in hotel quarantine tested positive after returning from overseas.

There are nine active cases, some linked to an outbreak on the Patricia Oldendorff bulk carrier off the Port Hedland coast.

Mr Cook said the infected crew, some of whom remain on the vessel with the rest in hotel quarantine, were recovering and the ship would depart soon.

"By splitting the crew into two different cohorts it's enabled us to really lower the viral load on board the ship," he said.

"That's enabled us to be in a much more confident position. The deep clean of the vessel will take just 24 hours and we'll be able to get it on its way as soon as possible."

Australian Associated Press