The Coalition's refusal to submit its policies for costing unless the source of a Treasury leak is investigated makes them a "laughing stock", Treasurer Wayne Swan says.
The opposition expects to see a $2.44 billion budget saving from scrapping Labor's National Broadband Network because of the interest it would not have to pay.
However, Treasury analysis, dated July 5, cast doubt on the figure, saying the savings would only be $1.6 billion, leaving an $800 million discrepancy, Fairfax reported on Tuesday.
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Coalition finance spokesman Andrew Robb vowed on Wednesday to withhold 20 policies from budget costings until the source of the leak was investigated.
Mr Swan said the opposition had only submitted about 10 per cent of its almost $30 billion in promises to the finance department and Treasury for scrutiny.
"That says to me they have got a cost blowout," he told reporters in the northern Queensland town of Mackay.
"That says to me they are a threat to the surplus."
Mr Swan said the coalition had "become a laughing stock" and Mr Robb's comments were "childish".
