A resurgent iron ore price has paved the way for a predicted $1.2 billion surplus in the state budget.

WA's finances are forecast to remain in the black over four years as its Labor government embarks on a debt-fuelled $27 billion infrastructure spend.

But Treasurer Ben Wyatt admits fortunes could quickly come undone should a fresh surge in infections eventuate.

A second wave of coronavirus would wipe $5.6 billion and 30,000 jobs from the economy.

"That is a huge risk," Mr Wyatt said.

"It would slow down the capacity for the state to deliver on its infrastructure plan and it would also slow down the capacity of the private sector to do the same.

"The impact on confidence would be dramatic."

Prime Minister Scott Morrison points out WA was set to receive a $1.5 billion top-up payment from the Commonwealth and has sent Premier Mark McGowan a tongue-in-cheek message in response to the surplus.

"The top-up payment, as a result of the GST reforms that the finance minister and I worked together on as treasurer, means that the Western Australian budget is in surplus," he told reporters in Canberra on Friday.

"And so you're welcome, Mark."

Western Australia has now gone almost six months without any community spread of coronavirus.

The budget assumes the state's hard borders will remain closed until April 1 - the same date nominated by federal treasury.

But Mr Wyatt says the date is just a projection and West Australians should not assume a decision has been made.

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

Both Victoria and NSW continue to record mystery cases.

Opposition treasury spokesman Dean Nalder labelled the budget insipid, saying it contained very few new announcements.

"The fact that WA Labor would run a surplus at a time when so many small businesses and households are suffering is appalling," he said.

Australian Associated Press