This article is more than 1 year old

This article is more than 1 year old

Labor has revealed its planned changes to the superannuation system will reap an extra $30bn over a decade, as the two parties continued to trade blows over budget costings on Wednesday.

Campaigning in marginal seats in Perth, the opposition leader, Bill Shorten, was forced to clarify the party’s superannuation policy, saying he had “misunderstood” an earlier question about proposed tweaks to the system.

“I thought I was being asked if there were unannounced changes to superannuation and we’ve already made the announcements of the changes we’re going to make,” Shorten said.

In 2016, Labor estimated its four major policy changes to the superannuation system, which are centred on the winding back of tax concessions on contributions, would raise $18.9bn over the decade. The Coalition had released figures suggesting the measures would accrue $34bn.

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On Tuesday Shorten said the party had no plans to raise taxes on super, but was forced to backtrack on the remarks when confirming Labor remained committed to its 2016 policy.

The prime minister, Scott Morrison, targeted the opposition leader for his slip-up, labelling him “sneaky and tricky”.

With Treasury also releasing its Pre-election Economic and Fiscal Outlook on Wednesday both Labor and the Coalition used the document to ramp up their respective attacks on the other’s costings and economic credibility.

Labor said the economic update confirmed a Grattan Institute report that suggested the government would need to reduce expenditure by $40bn a year by 2029-30 if it was to meet its spending cap as a proportion of GDP of 23.6%.

The opposition said the gap would need to be met with spending cuts, with treasurers from Labor states calling for the government to guarantee no further health and education cuts.

“They should have the moral fortitude to fess up as to what the cuts are and then we’ll have a proper budget debate,” Labor’s treasury spokesman, Chris Bowen, said.

“Of course, the economy will be front and centre in this election. We relish that. We have alternative plans for the economy and for the budget and we invite the government to be truthful with the Australian people about what their cuts are.”

Spending as a proportion of GDP is at 24.9% this year and is forecast to reduce to 24.5% by 2021-22 and 23.6% by 2029-30.

The Coalition says the published expenditure as a share of GDP for 2029-30 was a “projection and not a target” and reflected a moderating rate of spending growth over the next four years.

Facebook Twitter Pinterest Scott Morrison on the media bus, west of Devonport in Tasmania, on Wednesday. Photograph: Dominic Lorrimer/AAP

Asked about the $40bn difference, Morrison said the Grattan Institute “was not Treasury”.

“There are plenty of people who have lots of opinions on what these figures are but it’s actually the Treasury that combine these figures and they do that independently and professionally.

“It’s very shifty what we’re seeing Bill Shorten doing, just like he did at the last election, and I think people are wise to Labor’s shiftiness.”

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The Pefo document released on Wednesday, which is released before federal elections to update budget figures, showed no major changes to the forecasts published a fortnight ago.

Treasury also confirmed that the tax measures announced in the budget that would give immediate tax relief to low and middle income earners will need to be legislated before 1 July, meaning parliament will most likely return in June. The new Senate does not sit until 1 July.

Treasury said that if the changes were not legislated prior to 1 July 2019, the revenue cost of the measure would need to be reassessed.