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The Turnbull government’s budget forecasts will blow out by $9.6bn over the next four years, or $44.7bn within a decade, if IT fails to clinch parliamentary approval for stalled savings measures, new figures show.

A report by the parliamentary budget office reveals the cumulative impact of unlegislated measures that are already factored into the budget bottom line.

Wednesday’s report has emboldened the government’s political opponents, who argued it showed the flimsy state of the deficit forecasts and also proved many Tony Abbott-era cuts remained on the agenda.

But the figures also illustrate the Coalition’s difficulties in the Senate, as the government considers changes to electoral laws to counter the impact of micro-parties before the next election. Malcolm Turnbull said last week the government was trying to make “prudent” savings but “one of our challenges is that so many of them are being blocked in the Senate”.

Scott Morrison’s budget update in December predicted an underlying cash deficit of $37.4bn this financial year, with a cumulative shortfall of $108.3bn over the four-year cycle. But these deficit figures would be $9.6bn worse over the four-year period if the treasurer’s mid-year economic and fiscal outlook had not assumed the passage of a raft of unlegislated savings measures.