Treasurer Jim Chalmers says the windfall budget surplus forecast for 2023-24 will be about 50 per cent bigger than expected eight weeks ago, possibly gifting Labor the largest back-to-back surpluses on record.

The budget was forecast to be $9.3 billion in the black last financial year after a $22.1 billion surplus in the year before. But Dr Chalmers on Sunday said the final outcome due on September 22 would be much larger.