Treasury has uncovered a "black hole" in the coalition's costings of at least $7bn, independent MP Tony Windsor says.
It's put a significant question mark over whether he can support the coalition, saying it comes down to an issue of trust.
"We probably understand now why (Opposition Leader Tony Abbott) wasn't interested in releasing the numbers," Mr Windsor told ABC Television.
He and fellow independents Rob Oakeshott and Bob Katter met with Treasury secretary Ken Henry for briefings on costings from both Labor and the coalition.
While Labor's promises were "about on line", there was a deficit of between $7 and $11 billion in the coalition's bottom line, Mr Windsor said.
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Mr Windsor said the revelation makes him suspicious of the coalition, given its long refusal to submit its costings to Treasury, and is seeking an explanation.
"And essentially what we're after - well, I'm after - is a judgment on two different teams that want to be the government for the next period of three years," he said.
"One of those things that we have to establish is trust in what they're actually saying."
He expects opposition treasury spokesman Joe Hockey to go back to Treasury on Thursday to clear up the figures.
Mr Windsor said the coalition and Treasury were engaged in talks until late on Wednesday, pushing back the meeting with the independents.
Labor jumped on the finding as proof of why Mr Abbott tried to avoid Treasury scrutiny.
"This costings shambles is damning evidence of the risk the Liberals pose to the budget and to the economy," a spokesman from Treasurer Wayne Swan's office said in an email.
But Mr Abbott was standing by his figures, and blamed the use of different models and data for "several differences" in Treasury's assessment.
"This is a difference of opinion with Treasury," opposition finance spokesman Andrew Robb told ABC Radio.
The coalition will make good on its promise to deliver an $11 billion boost to the budget bottom line over four years, a statement from Mr Abbott's office says.
Even under the worst case scenario as outlined by Treasury, the coalition was still $7 billion better than Labor's bottom line over four years, the statement says.
"We remain confident that we will deliver on our commitments," it reads.
