National's Steven Joyce claims Labour has an $11.7 billion hole in its spending plans, Labour finanace spokesman Grant Robertson accused Joyce of being desperate and misleading the voters.
Joyce said he had found five errors in Labour's fiscal plan, in which it has set out its spending plans for the next four years and forecast the impact that would have on debt and surpluses.
He said they totalled $11.7 billion - which would result in higher debt than Labour had accounted for and Labour would have to increase debt by $20b on current levels - not the $7b it had claimed.
"That level of spending and increased debt can only lead to one thing - higher interest rates for Kiwi mortgage holders," Joyce said.
Labour leader Jacinda Ardern was sticking by the fiscal plan, saying Labour had it independently assessed by BERL to ensure it was robust.
The biggest error Joyce claimed was a failure to include the operating allowances for each year in the subsequent year's tallies - he said those figures should be cumulative.
"When operating expenditure is added, for example an increase in wages for Police, that expenditure continues into following years. Labour's operating allowances don't allow for that."
He said that meant a total of $9.4b was not accounted for over the four year period.
Robertson has slammed the claims by Joyce, saying it was "a desperate and cynical attempt to create a diversion."
Labour 's costings were robust and had been independently assessed.
"This is a desperate act from a flailing Finance Minister. What he is trying to do is disingenuous. He knows that we have accounted for our expenditure in health and education going out into future years.
"He is trying to, I believe, mislead the New Zealand public. I'm sure they will see through it."
Labour is planning to spend the bulk of future operating allowance on increases in health and education - $6.7 billion in health over four years and $1.8 billion in education.
In its fiscal plan, it has set that down as committed expenditure rather than including it in the operating allowance. Robertson said at the heart of the dispute was semantics about the definition of an operating allowance.
"What Mr Joyce has done is taken what is effectively our left over cash at the end and then tried to accumulate that out as if we are going to spend every dollar of it every year. I have never said that. I have never said that our fiscal plan did that."
However Robertson did concede that with the operating allowances it was allowing for areas of Government other than health and education - $835 million in the next financial year; then $879 million; $1.5 billion and $3.4 billion - 'we will be looking for departments to reassess their spending" and said Labour would have some "different priorities than the Government does in those areas."
"Like any Government we will make those choices when we get into Government."
Joyce also claimed Labour had failed to allow about $289 million for its policy to extend paid parental leave to 26 weeks - which Ardern said was included in the $4.7b for its wider "Families Package".
He also claimed Labour had double counted $902m in expected revenue from taxing multinationals - which had already been included in Treasury's pre-election update - and had undershot its costing of the first stages of the Families Package.
It had also failed to account for the finance costs which would be associated with extra borrowing, which Joyce said would cost $580m.
Ardern said Labour knew it could not roll out the social programme she planned unless it had a robust economy so had made sure its plans were sound.
She denied Labour had a record of bad management, saying Labour had run surpluses every year between 1999 and 2008.