National appeared to oppose the proposal yesterday, though ministers gave different views on the issue.
Prime Minister John Key said it was "not a terribly good idea". He said it would require a funding boost for the Treasury and would not achieve the Green Party's goal of greater transparency because the results would be manipulated for political gain.
"They would just ignore it if they didn't like the numbers," he said.
That contrasted with comments by Acting Finance Minister Steven Joyce, who said National "did not have a strong view either way" on the policy.
Mr Joyce said that if all Opposition parties were interested in it, then National would consider it.
"I'd say we'd be open to it, but let's see what other Opposition parties think," he said.
Labour leader Andrew Little said he supported the idea because it would improve transparency and help parties to understand the impact of the Government's policies.
Act Party leader David Seymour said it was a politically smart move by the Greens because it would allow the party to "sanitise itself in the eyes of the business world". But it would have problems in practice because the Government department might not be able to provide the definitive numbers the Greens were seeking.
Incoming Business New Zealand head Kirk Hope said the policy would make it easier and better for businesses to understand the costs of party policies. He said the system was already used in other countries.
"It's not something that is new or unusual and it could make a very useful contribution to be able to analyse policies."
Parties are already able to request the assistance of a full-time Treasury official for policy costings, but must pay for it out of their parliamentary budget.
Most parties opt not to use this resource, preferring instead to outsource their costings to private firms.
The Green Party estimated the new unit would cost $1.2 million a year, rising to between $2 million and $3 million in election years.
Eye on promises
• Greens want a new Treasury unit which audits all political parties' policies.
• The resource would be available to all parties and bound by confidentiality rules.
• Estimated cost of $1.2m/year and up to $3m in election years.
