Simon Bridges says he does not support the Government's proposed "fiscal watchdog" because he sees it as an opportunity to "screw the scrum" on the Opposition.
The National leader said he doesn't trust the Government on the proposal, which would see an independent unit within Parliament to analyse the financial fitness of political party policies.
That's despite Finance Minister Grant Robertson saying National's former finance spokesperson Amy Adams welcomed the proposal when he mentioned it to her earlier this year.
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Bridges revealed why he doesn't trust the Government. He said since becoming Leader of the Opposition, he's been trying to get a Treasury secondee to help him with the costings of National's economic policies.
A Treasury secondee is essentially an independent number-cruncher. It's the chance to have a Treasury staff member oversee the legitimacy of financial policies.
"Since being the Leader of the Opposition, I've tried really hard to have a Treasury secondee official to help me with costings with our economic policies and holding the Government to account," Bridges said on Tuesday.
"I feel like we've been obstructed from the get-go, by both the Minister of Finance and also the Treasury."
Bridges said he even offered to have the secondee in his office "at the Opposition's own cost".
"If they won't even provide me with a secondee at the Opposition's own cost, how can I trust them with a supposedly independent institution over the top of that to provide a view on our costings and the like?"
The Finance Minister responded by saying he was "hugely" disappointed that Bridges didn't support the proposed "fiscal watchdog" - a term suggested by the New Zealand Initiative who proposed the idea in 2014.
"The proposal that's been put forward is in fact for an Officer of Parliament position, so that really is a high level of independence," Robertson said on Tuesday.
"I think it's actually really important for the way our democracy works to be able to have scrutiny of both the Government's actions - which is what this body will do - but also the promises of political parties.
"It's hugely disappointing Simon Bridges doesn't want that kind of scrutiny."
Robertson said Bridges' claim that he's been refused a Treasury secondee was "not accurate at all", and said that is "not the function of a Treasury official we're talking about here".
"That's actually a completely separate debate around whether or not, as has happened in the past, Opposition offices get a secondee," Robertson added.
"I didn't manage to get one for quite a while [when he was Opposition Finance spokesperson] and he'll need to take it up with the Treasury as to exactly what's happening on that."
The Government hopes the proposed independent body would leave limited room for debate over the financial outcomes of political party policies.
For example, former National Finance Minister Steven Joyce made the claim, ahead of the 2017 election, that Labour had made an $11.7 billion error in its policy analysis.
Labour staunchly denied they had a "fiscal hole" as Joyce claimed, and more than a year on from the election, it was revealed the Government was sitting on a surplus of $5.5 billion - $2.4 billion more than forecast.
Setting up the entity delivers on the confidence and supply agreement Labour has with the Greens, who estimate it would cost $1-2 million per year, increasing to approximately $2-3 million in an election year.
Robertson said the new unit wouldn't be established in time for the 2020 election.
In the meantime, Treasury will establish a new team to provide a policy costing service to the political parties currently represented in Parliament.