National is calling on Labour to release the costings on all of its agreed policies with New Zealand First and the Green Party.
Finance Minister Grant Robertson appeared on both The Nation and Q+A programmes this weekend. He told The Nation that Labour looked carefully at the commitments it was agreeing to in negotiations, "and made our best estimate of the costs".
"Obviously, when you're in Opposition, you have only a certain amount of resources to do that. We are absolutely confident that we can meet the expenditure that is in there and actually still meet our budget responsibilities.
"We now have the ability to work with the public service to refine the estimates that we've made. But I can give you an assurance that it fits within the confines of our budget responsibility rules."
Today, National's finance spokesman Steven Joyce said all parties involved in post-election negotiations had access to the public service to cost their commitments.
"So that excuse just doesn't wash. That sounds like someone who simply doesn't want to reveal the numbers," Joyce said.
"He's either had them costed and doesn't like what they add up to, or not had them costed...[voters] are entitled to know how much has gone out of their collective pockets in the process of forming this Government."
Labour and the Green Party released budget responsibility rules ahead of the election, in an effort to reassure New Zealanders about the economic approach of an alternative government. The rules include delivering an operating surplus, reducing net core Crown debt to 20 per cent of GDP within five years of taking office, and maintaining the current track of core Crown expenditure.
In announcing he was backing Labour, NZ First leader Winston Peters spoke of a future economic slowdown, warning of "dark days ahead".
Robertson told The Nation he was more optimistic.
"When we wrote the budget responsibility rules, we wrote into them the fact that if there was an economic shock, we would look to have surplus across an economic cycle."
Labour will hold a caucus meeting on Tuesday morning, followed by a Cabinet meeting. Labour has pledged to implement its 17 policy priorities - such as free tertiary education - in its first 100 days.
That gives it until February 2. Tuesday's Cabinet meeting will consider whether a mini-Budget could help get through legislation.
A mini-Budget is not a legal requirement but if a Government is making many major changes, including switching existing authorised expenditure to another area, it could be done as a package to give it a greater sense of coherence.
Robertson said he would produce a half-year economic fiscal update before the end of the year, as required under law.
"There are some things we want to do immediately that have a fiscal impact. An example of that is we've committed to restarting contributions to the New Zealand Super Fund immediately.
"There's a series of things that we've got to do. Whether we end up calling that a mini-Budget, we're working on that right now."