Wednesday’s mid-year budget update will downgrade company tax receipts  by $8.5 billion over the four years to 2027-28, and show “slippage’ in the bottom line in some years of the forward estimates, Treasurer Jim Chalmers says.

The company tax downgrade is the first since 2020 and reflects weaker  volumes in commodity sales  as the Chinese economy faces problems. Mining exports will be downgraded by more than $100 billion  over the four years to 2027-28.

Chalmers said "the global economy is uncertain, the global outlook is unsettling and that’s weighing heavily on our economy”.

The budget bottom line is also being hit with extra spending in a range of areas.

The update will include an extra  $1.8 billion in payments to veterans’ “as we  clear the Liberals’ claims backlog.”

Chalmers said on Sunday the update included “a number of spending pressures in areas that are very important to us, which are putting substantial pressure on the budget”.

Apart from the spending on veterans, the treasurer  said there were a handful of other “very big estimates variations”, including in disaster funding, early childhood education, Medicare and some other areas.

“A combination of these things, slowing growth, a write‑down in mining exports, a write‑down in company taxes, combined with upward pressure on spending in these important areas means that there will be some slippage in some of the bottom lines over the forward estimates – even as we work very hard to get the bottom line in the current year into slightly better nick,” he said.

Chalmers also said he would announce the members of the two new Reserve Bank boards on Monday or soon after.

The opposition has accused him of wanting to stack the monetary policy board that will set interest rates.

Chalmers has offered all members of the existing Reserve Bank board a place on the monetary board and most will transition to it, although there will be room for some new members. This left a number of vacancies on the new governance board for him to also fill.