The finance minister is accusing Simon Bridges of "electioneering" and introducing "the very worst of American politics'' to New Zealand.

Photo: RNZ / Dom Thomas

The Opposition leader said the decision to set up a new watchdog to cost political parties' election promises is simply an attempt to try to "screw the scrum" against National.

Finance Minister Grant Robertson said Simon Bridges' criticism of an independent office made no sense.

"It's absolutely ridiculous, we're talking about a proposal for an officer of Parliament here. It's like saying that the Ombudsman is somehow screwing the scrum.

"This is the ultimate level of independence that could be offered in the system," he said.

Mr Bridges was not having a bar of Cabinet's recommendation for a Parliamentary Budget Office (PBO), which would have oversight of policy costings put up by political parties during election campaigns.

He said he would block it every step of the way, because he did not trust the government.

"They want to illegitimately, undemocratically screw the scrum on the opposition," he said.

But Mr Robertson labelled that kind of narrative from the Opposition leader as the very worst of American politics - something he says New Zealanders have no interest in.

"This feels to me like political gameplaying, potentially electioneering. I think it's the introduction of a style of politics into New Zealand that New Zealanders don't want.

"We don't want to be taking the very worst of American politics or the very worst of the Crosby Textor playbook again. That's what this feels like from Simon Bridges,'' Mr Robertson said.

The Green Party co-leader James Shaw isn't surprised by Mr Bridges' rhetoric saying it's just a continuation of his attack on Statistics New Zealand last week when he questioned its credibility and work after a review laid bare significant data gaps in the 2018 census.

Mr Shaw said the Opposition leader is set on undermining public institutions.

"It's really consistent with everything Simon Bridges has been doing recently which is to try and undermine public confidence in public institutions, especially independent objective institutions that are designed with upholding the quality of our democracy. So he's had a real go at those sorts of institutions recently and this language is consistent with that,'' he said.

Mr Robertson said the independent officer position can't be progressed without National's support.

"I hope with a deep breath National can sit down and take a look at this proposal and see that it's actually good for political parties and good for the public,'' he said.

If National doesn't get on side then Mr Robertson said the government will look at other ways of doing it.

Mr Bridges told Morning Report today the reason he was opposed to it was he did not trust the government on electoral matters.

"The convention has been for a long time - and we've adhered to it very strictly over the last nine years of the National government - that on electoral matters you get consensus or near consensus, you certainly have the opposition on board.

"They've taken referendums out of Parliamentary control and they've taken them to Cabinet ... they've made a bunch of changes - they're seeking to - such as same-day voting on election day without recourse to us or our views on these things."

He implied the move was an attempt to gain political advantage by gaining access via Treasury to National's policy plans ahead of time, despite Cabinet having recommended setting up an independent Officer of Parliament - like the Ombudsman's Office and the Auditor-General and the Parliamentary Commissioner for the Environment.

"Grant Robertson wants it for Treasury at the next election. Well, that's nice in theory but I'm sorry in practice as Opposition Leader I'm not going to hand over in advance in the heat of an election campaign my deepest secrets and strategies and policies beforehand to Treasury."

The Green Party had initially suggested in its 2016 policy to institute a policy costing unit inside Treasury, but the government's statement yesterday clearly said it was recommending the PBO be an independent Officer of Parliament, and did not expect it to be operational until 2021 - after next year's election.

"Part of my distrust ... was the fact that Grant Robertson is seeking to do this while on the other hand I spent months and months and months begging him for a Treasury official so that we could do proper costing so that we could hold the government to account on the Budget and he played politics with that," Mr Bridges said.

It's understood that there was a delay for National to be offered a Treasury Official, but one was offered and National rejected that person saying they were not what the party was looking for.

Mr Bridges said Mr Robertson and Mr Shaw were "rattled" and "getting nasty earlier than I expected".

"There was personal attacks on me. I understand that, they're rattled, they're under pressure," he said.