National says Labour is exploiting a loophole by sending out taxpayer-funded material adorned with its MPs' smiling faces that doesn't have to be declared as election spending.
A number of Labour's MPs have sent the 50-page "information kit for the over 60s" to their constituents.
Each kit carries the name and photograph of the MP sending it, but no party logo, and uses a orange colour rather than red.
Its information is functional but touches on policy successes of the Labour-led Government, such as the SuperGold Card.
The parliamentary crest on the front of the booklet shows it has been paid for by the Parliamentary Service.
Mt Roskill MP Phil Goff, for example, has printed 4000 of the booklets at 91c each - a cost of $3640. The postage was also paid by the taxpayer.
The booklet has been made available to all Labour MPs but the Parliamentary Service would not say how many had been printed or at what cost, directing the Herald to ask each individual MP. At least four other MPs are distributing the kit.
As the booklet is not deemed an advert by the Chief Electoral Office, the costs do not count against the $20,000 limit set down by the Electoral Finance Act for each candidate.
National deputy leader Bill English said it was a "loophole" that created an unfair playing field for new candidates up against MPs.
He said National felt so strongly that its MPs were not sending out any material using their parliamentary budget.
Mr Goff said the booklet had been cleared by the electoral office, and since sending it he had received "one of the most positive responses I've had in terms of my constituents seeking information in 25 years".
"It's not a loophole. MPs don't stop serving their constituents during the election period. All MPs provide their constituents with information, such as what they are entitled to, from the day they are elected through to the day of the next election. This has always been the case."
Chief Electoral Officer Robert Peden said he had assessed the booklet, and "it does not contain words or graphics that could be reasonably regarded as encouraging or persuading voters to vote, or not to vote, for a candidate".
Mr Peden said it was part of each MP's "constituency function" rather than his or her candidacy in the upcoming election.