A confidential Treasury analysis has revealed an $800 million hole in the Coalition budget costings Tony Abbott has promised will add up to a bigger surplus in three years than that promised by Labor.

The Coalition's single biggest budget saving - $2.44 billion over the next four years - comes from the interest it will not have to pay when it scraps Labor's national broadband network.

But, according to the Treasury analysis dated July 5 and obtained by the Herald, the savings on interest payments would be $1.6 billion, a discrepancy of $800 million.

With the Coalition preparing to unveil a bigger budget surplus at the end of the campaign as a ''grand finale'' to its claim to be a better economic manager and the government using an economic attack against Mr Abbott to try to turn around its campaign, the battle over budget costings is expected to be fierce.

The Opposition Leader repeatedly refused yesterday to put a figure on his claimed budget surplus for 2012-13, but pledged it would be ''significantly larger'' than Labor's figure of $3.5 billion.