The National Party says the Government's planned independent policy costing unit is "screwing the scrum" and leader Simon Bridges wants no part in it.

Moments after the policy was unveiled by Finance Minister Grant Robertson, Bridges rejected the idea because he "does not trust the Government".

Robertson said the lack of support from National was "hugely disappointing" and he accused Bridges of "playing politics".

This means the future of the unit – which received the green light from Cabinet yesterday – is unclear because it would have needed Opposition support to be set up.

Finance Minister Grant Robertson said ministers have recommended an independent Parliamentary Budget Office (PBO) be set up to cost party policies before elections.

Such an entity was in the Labour/Green Party confidence and supply agreement.

But the PBO would not be established until 2021, meaning it would not be in place before next year's election.

Robertson said it would have been ideal to have it set up sooner, but the Government was "taking the time to get this right".

However, Treasury will establish a new team to provide a policy costing service to the political parties, currently represented in Parliament, before the election.

Such an independent body was called for after the now-infamous "fiscal hole debate" before the 2017 election.

Associate Finance Minister James Shaw said the independent body would mean more transparency around what political parties are promising to do.

He said there would be "fewer political games played" as a result of the unit.

Treasury documents investigating the establishment of such a unit, released earlier this year, said recent elections have seen a significant focus on fiscal policy and the cost of election platforms.

In a press release, Robertson had sounded optimistic about National supporting the policy.

"We engaged with the National Party on our proposal earlier in the year, and welcomed former finance spokesperson Amy Adams' positive response to the proposal," he said.

"We understand that National's new finance spokesperson Paul Goldsmith will be consulting his caucus on the proposal."

However, Bridges this afternoon made it clear the policy would not be supported by National because he "did not trust the Government".

The independent Parliamentary Budget Office (PBO) was planned to be set up to cost party policies before elections.

This would, according to Associate Finance Minister James Shaw, mean "fewer political games" would be played before the elections.

Part of Bridges' opposition to the entity was based his claims that Treasury had refused to provide National with an official who could cost its policies.

It is understood Treasury asked its officials if anyone could cost National's policies but, at first, no one wanted to do it.

Someone was offered up but National rejected their help, it is understood.

A spokeswoman for Bridges said that after months of asking for a Treasury official, National was offered one person who it didn't find to be suitable for the role. 

Robertson said it was "hugely disappointing" that Bridges would not be supporting the creation of the new unit.

"It's really sad to see Simon Bridges playing politics when we're actually trying to increase transparency for voters."

The lack of support from National creates a problem for the planned unit.

Without the Opposition support, it is very unlikely to be set up.

Robertson said what happens next is up to the Speaker of the House, Trevor Mallard.

The creation of a PBO was part of the Labour/ Greens supply and Confidence agreement.

Treasury documents investigating the establishment of such a unit, released earlier this year, said recent elections have seen a significant focus on fiscal policy and the cost of election platforms.

A notable example of this was last election's $11.7 billion "fiscal hole" saga, where National claimed Labour's policy numbers did not add up.

Labour denied this.

With an independent costing unit, the risk of such a debate would be reduced. 
PBO would not be established until 2021, meaning it would not be in place before next year's election.

Robertson said it would have been ideal to have it set up sooner, but the Government was "taking the time to get this right".

However, before the election Treasury will establish a new team to provide a policy costing service to political parties currently represented in Parliament.