First home buyers would be able to withdraw money from superannuation for a deposit on the condition they put it back later and save more quickly for deposits by having their pay directed into superannuation-like accounts under proposals being considered for next Tuesday's budget.

Scott Morrison and Malcolm Turnbull's second budget is set to forecast a surplus in 2020-21 using a new measure of budget balance called "net operating balance" that will exclude extra spending on infrastructure, however Australia's preeminent budget forecaster questions this, saying it won't return to surplus for the foreseeable future.

Deloitte Access Economics, the so-called "treasury in exile" run by former budget official Chris Richardson, says that forecast looks doubtful because it assumes tens of billions of dollars of cuts in projected grants to the states will stay in place and that the Senate will pass cuts it has shown no interest in passing.

"The budget simply won't whirr back into surplus," he said ahead of the publication of the Deloitte Access Budget Monitor on Monday.