Former Labour leader Phil Goff was grilled by National in 2011 over the affordability of policies. Here he is with Simon Bridges in 2017. Photo / Nick Reed
A proposal for an independent but state-funded body to assess the costings of political parties' policies is the result of a few lessons which were hard learned by Labour.
Those lessons were stark in the 2011 election when Key pinioned then leader Phil Goff with his "Show Me the Money"line, questioning how Labour would afford its policies.
This was reprised with equal effect in 2017, when Steven Joyce popped up his claim of a $11.6 billion hole in Labour's costings.
That claim – whether true or not - put the handbrake on Ardern's momentum by creating doubt in voters' minds.
The other lesson was the trouble the party had in selling policies such as KiwiBuild in two successive elections because there was (well-founded, as it turned out) scepticism about the capacity to deliver on it.
It was the Green Party's idea to set up a unit to cost policy promises, but one Labour supported having struggled with that through its nine years in Opposition.
It should technically have benefits to political parties, especially those in Opposition who do not have the same resources or staff to undertake such work.
It would also have benefits to the voters, who would have more than a politician's word to rely on in assessing the viability of those policies.
They could have some faith in whether politicians could actually deliver on what it was promising.
So it may surprise some that the immediate response to Robertson's announcement of the Parliamentary Budget Office was National Party leader Simon Bridges accusing the Government of trying to screw the scrum, and Robertson accusing Bridges of going all Australian-style attack politics out of spite.
All the hot air has rather diminished the genuine pros and cons around such a body.
In that regard, theory and practice are two different things.
It would be up to political parties to decide when and whether to put in their policies up for scrutiny.
Logically, that would be before they were announced publicly. That would allow a party to decide whether it needed to amend (or scrap) a policy before the public had seen it.
Having it costed after a policy was announced could result in the embarrassment of having to subsequently water it down or admit it was unaffordable.
However, that is expecting political parties to trust an external body with highly sensitive and confidential material: campaign promises.
Politics and paranoia go hand in hand, and with good reason.
However independent such a body might be in structure, where there are humans there are risks of leaks.
National will not be the only party wary of using the unit because of that.
It is hard to imagine NZ First calling on it either.
That party had to let it pass through Cabinet because it was contained in the Green Party's support agreement.
But it does not have to use it.
NZ First leader Winston Peters is highly unlikely to hand over a ream of his policies for other eyes to scan through.
Nor is it necessarily in his interests to have his party policies costed: in fact, knowing the cost could actually be a liability.
Proper cost estimates are effectively considered a necessity for the two large parties: National and Labour.
But the smaller parties have more leeway to promise the world, knowing full well they might have to settle for a small atoll once the compromises of coalition talks take effect.
Much of this to-ing and fro-ing is simple political bickering.
Bridges' antagonism is partly fuelled by sour grapes over his efforts to get a Treasury secondee into his office.
National, perhaps a tad paranoid by now, was apparently first told nobody wanted to do the job and then offered someone National did not consider had the appropriate skills.
But National does have two other, larger reasons for objecting to the costings unit. One is purely political, and one is on principle.
The objection in principle is that it is a step toward greater state funding of political parties.
National has long believed parties should be able to fund themselves, while the Green Party in particular has advocated greater state funding to reduce the need to rely on donations.
The political reason is that it will remove a powerful election campaign strategy for the National Party: the tool of being able to cast doubt on their rivals' policy costings.
In that regard, the prospect of having their own policies assessed by outsiders is nowhere near as bad as the prospect of their rivals being able to do so.