A key independent MP says Opposition Leader Tony Abbott needs to explain why Treasury analysis has identified a hole of up to $11 billion in the Coalition's election promise costings.

Before the election the Coalition said its promises would add about $11.5 billion to the budget bottom line over the next four years.

But Treasury analysis given to Tony Windsor and his fellow independents Rob Oakeshott and Bob Katter shows the Coalition's promises would only add between $860 million and $4.5 billion to the bottom line.

The Treasury document shows the Coalition has costings problems with its plans for health, education, infrastructure and its paid parental leave scheme.

The difference in figures also comes down to the Coalition making significant spending promises without saying which projects would be slashed to pay for those promises.

The three independents have now received Treasury briefings on the impacts of the election promises made by both the Coalition and Labor.

Mr Windsor says Opposition Leader Tony Abbott needs to explain the "black hole".

"There's some questions that the Coalition will need to answer in terms of the magnitude of the black hole in their promises and funding arrangements," he said.

"And the Coalition commitments at the election - there's something like an $11 billion black hole under Treasury assessments.

"It is an enormous amount of money. The budget bottom line doesn't change substantially but the commitments that were given and the costings that were given going into the campaign, or going into election day, are substantially different when they were assessed by Treasury officials."

Mr Abbott had originally refused to submit his costings to Treasury because of a leak.

Mr Windsor says the Treasury's analysis probably explains why Mr Abbott was cagey with his figures.

"We probably understand now why he wasn't interested in releasing the numbers and so I think we all await an explanation of where these significant differences are in terms of interpretation," he said.

"And I think Andrew Robb came up with some various reasons - legal reasons, the leak and the breach of caretaker provisions."

But opposition finance spokesman Andrew Robb says the Coalition is standing by its costings.

"We will improve the Budget bottom line by more than $11 billion if we do get the opportunity to form government," he said.

"On top of that there's another $22 or $23 billion of cuts in the debt that will come from capital savings, none of which was challenged [by Treasury].

"There was a difference of opinion with Treasury. But it doesn't necessarily represent a mistake - it's a difference of opinion."

Mr Robb says the Coalition's worst-case scenario is that they are $7 billion better off than Labor.

"We know there's billions of dollars which will cover what we say we will spend and Treasury acknowledged that," he said.

"We used slightly different assumptions and slightly different data and it's just a matter of judgement."

Mr Robb says he is not worried about how this will affect the independents' decision on which party to support to form a minority government.

"They (the independents) will give us an opportunity to explain why this is a difference of opinion, not a costing error, and they will move on," he said.

Labor said its promises would improve the Budget bottom line by about $44 million over four years, but Treasury says it is actually more than that - about $106 million.

Mr Windsor says Labor's election promises were pretty much on target.

"[Treasury] costed the Labor Party's promises and they're about on line with what they said they would be," he said.

"[Treasury's] interpretation of the Government's election promises are that they're fairly squared off; there's very little difference between what they (Labor) said they would cost and what they would actually cost under the Treasury estimates."