Former "perkbuster" Rodney Hide is joining other political parties in fighting the Auditor-General's view that much taxpayer-funded party advertising for last year's election was unlawful.
Mr Hide was a stern critic of Labour's $446,000 pledge card, and in February urged Helen Clark to repay the money.
But now that Act's campaign spending is under scrutiny, Mr Hide is taking a different stance.
He has written to Parliamentary Service general manager Joel George saying the Auditor-General's draft finding has wide implications that must be considered.
"The ruling is pretty devastating," Mr Hide said.
"Essentially the Auditor-General has come along, looked at the pledge card - which is clearly way outside the rules - and then come up with a new rule which makes us all guilty."
Like several other parties, Act has refused to reveal how much of its spending has been placed under question by the Auditor-General's draft ruling.
Mr Hide has questioned National's decision to pay back its amount, and won't say if he will repay back his own total.
Act has also not decided whether it would support legislation to validate the spending if Labour opts to take such an action.
Among the examples of Act's pre-election advertising cited by the Auditor-General in his draft ruling were newspaper spreads two days before the ballot proclaiming "What Act Brings To Parliament", and listing things the party wanted.
Taxpayer money is not allowed to be used "for the purpose of supporting the election of any person or the casting of a party vote for any political party".
Mr Hide says Act's advertising is different to Labour's pledge card. "The pledge card is outside the rules. I'm within the rules."
He said he sought approval from Parliamentary Service for all his spending and "I don't know what else I can do".
Mr Hide has become known as parliament's perkbuster for his relentless attacks on MPs who tried to reap benefits from their position.
He was the only MP to vote against legislation allowing members to retain the right to fix their own allowances.
He was also responsible for the introduction of closer scrutiny of air travel perks for former and current MPs.
In February, he made a colourful speech in the House attacking Labour's pledge card.
"Helen Clark was caught red-handed using taxpayers' money - money they pay for a police force, for an education system, for a health system, not to fund the crooked Labour Party," he said.
Mr Hide has been backing away from his public image as a perkbuster in recent times, although he said yesterday that role had made him very careful about his spending. He had "bent over backwards" to be scrupulous and abide by the rules. But he acknowledged he had a problem explaining his current position to the public.
"There are two sets of rules - the rules we were all working under at the time of the election, which we complied with totally, and the Auditor General's rule."
Then and now
Helen Clark was caught red-handed using taxpayers' money - money they pay for a police force, for an education system, for a health system, not to fund the crooked Labour Party." - Rodney Hide, in February.
"There are two sets of rules - the rules we were all working under at the time of the election, which we complied with totally, and the Auditor General's rule."- Mr Hide, yesterday