The timing of West Australian treasurer Troy Boswell’s personal leave before the state budget is “curious”, the opposition says.

Premier Colin Barnett is stepping into the role of treasurer to cover for Mr Buswell, who has taken two weeks of personal leave for health reasons.

The premier will handle the treasury and transport portfolios until at least March 14.

Opposition spokesman Ben Wyatt says the timing raises “curiosity”, with the budget expected to be delivered in the first two weeks of May and the cut-off for many details to be determined by mid-April.

“What that means is all the key decisions surrounding the budget will happen over the next four-week period,” he told Fairfax radio on Tuesday.

“The treasury’s normally very, very busy, the under treasurer is normally very, very busy finalising the budget process, so I think that’s what’s got people curious.”

The government says the budget won’t be affected by Mr Buswell’s absence.

Finance Minister Mike Nahan will assist Mr Barnett in the budget process.

“Treasurer Troy Buswell will be on personal leave for the next two weeks. The reasons for this leave are health related,” Mr Barnett said in a statement.

Long-serving under treasurer Tim Marney resigned last month with his deputy, Michael Barnes, now acting in the role in the lead-up to the budget.

In 2010, Mr Barnett acted as treasurer while Mr Buswell went to the backbench after agreeing to pay back entitlements he claimed during a trip with former partner and Greens MP Adele Carles.