MPs dipped into the public purse to help campaign in the Mt Albert by-election by taking free flights to get there and attending unrelated meetings so taxpayers would pick up their hotel bills.
At least 17 MPs from outside Auckland took part in the four-week Mt Albert campaign.
Under Parliament's rules, they were free to use their right to unlimited domestic air travel for personal purposes, including electioneering.
But other expenses such as accommodation, taxis and rental cars are not covered if they are incurred while MPs are campaigning for their party.
The Herald has learned many of the campaigning MPs arranged other legitimate "parliamentary business" in Auckland at the same time, enabling them to claim for expenses such as accommodation.
National, Labour and the Greens used MPs from outside Auckland to campaign.
Yesterday, all three parties refused to give details of the official business that took their MPs to Auckland.
They would not give individual answers to a Herald survey asking when the business was arranged, how much time the MPs spent in Auckland and how much of that time was spent campaigning.
The Parliamentary Service, the department handling MPs' expenses, takes MPs at their word.
It is not subject to the Official Information Act, so the MPs' spending and travel during the byelection period cannot be subjected to public scrutiny.
MPs have unlimited free flights for personal purposes as part of their overall employment package.
There has been no call for restraint despite all other Government departments being told to cut costs because of the recession.
National whip Chris Tremain said his party's MPs "acted in accordance with the rules".
He said National understood the need for increased transparency on MPs' expenses and would follow the disclosure rules being developed by Speaker Lockwood Smith, but the party would not detail individual expenses for the Mt Albert byelection.
Labour whip Darren Hughes said the "vast majority" of its MPs were in Mt Albert during trips to Auckland on parliamentary business, but "it is not our practice to release our diaries".
He had reminded MPs they would have to pay their own costs if they were solely engaged in electioneering.
Greens co-leader Metiria Turei said Green MPs acted within the rules.
The party would release its MPs' total spending annually, and that was all the detail it would provide, as the "amount spent is the key issue".
National list MP Aaron Gilmore, from Christchurch, said he was in Auckland for an education meeting.
It had been arranged "weeks, not days" beforehand, and he had spent only a couple of hours campaigning.
Dunedin South Labour MP Clare Curran said her campaigning coincided with meetings and work related to her communications and information technology portfolio.
MPs use their flights and expenses in the same way during a general election campaign.
Parliamentary Service general manager Geoff Thorn said MPs were "honourable people" who were relied on to stick to the rules set down in the Speaker's directions.
Mr Thorn said MPs were sent a reminder about the criteria for claiming accommodation, rental cars and taxis during the byelection.
The service relied on the MPs certifying that their expenses were incurred on parliamentary business.
Because of the amount of travel MPs did, "the transaction cost of checking every individual trip simply doesn't make it worthwhile".
Mr Thorn believed there were no problems with MPs taking advantage of the system, and said many went out of their way to make sure they complied with the rules.
Otago University associate law professor Andrew Geddis said paying for campaigning trips from the public purse was state funding of political parties by stealth.
"This is not remarked upon when everyone jumps up and down saying political parties shouldn't get state funding for elections - they already do."
Professor Geddis said MPs' campaigning was a grey area which was difficult to address.
One way was to be "incredibly specific" about what MPs could do - which would lead to opposing parties "bludgeoning" one another with allegations of breaches.
The other option was to accept that MPs were "political beasts" as much as they were public representatives.
Professor Geddis said MPs' free travel could be suspended before general elections, but this would be more difficult for a byelection that took place alongside their routine work.
MPS IN MT ALBERT
The Herald saw at least 17 out-of-Auckland MPs on the campaign trail.
LABOUR HAD 12
Annette King, Darren Hughes, Maryan Street, Moana Mackey, Sue Moroney, Grant Robertson, Jacinda Ardern, Clare Curran, David Parker, Charles Chauvel, Grant Robertson and Clayton Cosgrove.
NATIONAL HAD THREE
John Carter, David Carter and Aaron Gilmore.
THE GREENS HAD TWO
Metiria Turei and Sue Kedgley.