Voters would be better informed, and Government-forming negotiations made easier, if we had an agency charged with providing impartial, independent costings of the policies parties put before us at election time.

It is too easy, as things stand, for politicians to go around promising the earth, and throwing in the moon for good measure, with barely a dollar sign to be found in their manifestos.

The OECD, in its latest country report on New Zealand, noted that the Treasury does not cost or assess opposition parties' policy proposals.

"Consideration should be given to strengthening New Zealand's institutional framework in this regard, possibly by allocating that responsibility to an existing agency such as the Treasury. Another option, which might enhance the perception of independence of the evaluations, would be for a new fiscal council to provide such estimates," it said, adding that most OECD countries have some form of independent fiscal institution.

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In a note titled "Keeping the Bastards Honest", Peter Wilson, a principal economist at the NZ Institute of Economic Research, puts forward some ideas on how that might be done.

Rather than have some permanent standing fiscal agency that would spend most of its time doing nothing while the relevant experts beaver away at the Treasury, Wilson suggests that a Parliamentary Commissioner for Policy Costings be appointed for a fixed term before each election. That commissioner would oversee and take responsibility for the work of officials seconded to his or her office for the purpose.

Such a person, perhaps a former Treasury Secretary or Auditor-General, would have the independence and mana of an officer of Parliament, while the political neutrality of the Treasury and wider officialdom would be preserved.

Wilson envisages a range of tasks a Parliamentary Commissioner might be called upon to undertake.

At the simple end, it could be to provide background information and guidance that parties could use when preparing their own costings.

Moving along the spectrum, the commissioner could, at the request of a political party, prepare costings of proposals which would remain confidential until the party announced the policy. Wilson says that in Australia and Canada, parties can request a costing of other parties' proposals, which increases the incentive on all of them to have their policies independently costed, knowing that if they don't, their opponents can.

Finally, a more proactive approach - which Wilson favours - would be to require the commissioner to prepare and make public ahead of polling day costings of the manifestos of parties contesting the election.

Clearly there would have to be some materiality threshhold, lest the system be clogged up by lots of single-issue or vanity parties with little prospect of election. It might be limited to parties already in Parliament - such transparency being one of the burdens of having made it there.

Whether you go for the reactive, on-request option or the proactive, comprehensive one will affect parties' behaviour.

Eric Crampton, at the New Zealand Initiative think tank, has argued that if costings are provided on a voluntary and confidential basis, parties will tend to seek the costings early in the election period and announce them when tactically advantageous. But they might confine such requests to policies they think will fare well under evaluation.

If, on the other hand, they are contemplating policies they think are good politics but bad economics, or if they know they will be costed, like it or not, they will have an incentive to delay revealing the policy, or to be as vague or as complicated as they can, to make the valuation as difficult as possible.

"Were all election promises costed by an independent agency, you would be well advised to delay submitting your policy for costing so that only a cursory take on it could be provided," Crampton says.

"You would also be tempted to wrap the policy in enough complexity that any costing provided in a hurry would have to make simplifying assumptions. Or conversely, you could avoid providing any of the details necessary for a costing, saying the finer points would be worked out while in office."

In these cases, the most we could expect from a Parliamentary Commissioner would be a qualified opinion or plausible range of costs, festooned with caveats, and leave it to voters to draw their own conclusions.

But we should not let the best be the enemy of the good, or the better.

In the meantime, what can the media (or what is left of it) do?

Some fact-checking is possible. For instance, when it was New Zealand's policy to drop the GST on food, it was one of the few policies it put a price tag on - $3 billion.

To see if that was plausible, we could look at the consumers price index basket as a rough proxy for the GST base, and how much of it is food (just under 18 per cent). And it turns out that 18 per cent of the GST take of $19.5b is $3.5b, so $3b was a respectable estimate when the only exceptions to the exemption would be for restaurant meals and takeaways.

When the scope of the policy suddenly shrank - to removing GST from "basic, essential foods" - the party estimated the cost at $600 million to $700m. With no indication of which foods those are, the best a fiscal fact checker could probably do would be to look at other jurisdictions like Australia and Canada, which exempt some foods but not others, and estimate what their rules would imply if applied here. An independent body with the resources to do that would be useful.

More broadly, we can make assumptions based on the statisticians' projections of population growth and the Reserve Bank's credibility on inflation and see if parties' fiscal forecasts plausibly allow for those two pressures. And of course, errors in those assumptions affect both the spending and revenue sides of the Crown's books, so there is a bit of a natural hedge there.

But the CPI is a poor proxy for cost pressures in, say, the health sector and both the magnitude and the composition of net migration gains matter for more granular estimates of pressure on the Budget's big-ticket items.

We would all benefit from more expertise and transparency in these things.