Prime Minister John Key has used taxpayer funding provided through Parliament to ask voters which political party they vote for, potentially providing canvassing data for the National Party.

The survey asks for voters views on a range of topics, but also asks which party they always, or usually, support.

It has sparked questions from the Opposition about whether the rules covering taxpayer-funded mailouts from Parliament should be reviewed.

The letter, which has landed in mailboxes across the Wellington region, has three inserts; a letter from Key, a summary of the budget, and a questionnaire for voters to fill out and send back Freepost to Key at Parliament.

All three inserts carry the parliamentary crest, and a spokeswoman for Key confirmed they were paid for out of the taxpayer-funded ''leader's budget''.

A Wellington Central voter, who asked not to be named, forwarded the personally-addressed letter to Fairfax Media saying she was concerned because the questionnaire would identify her political allegiances.

The Mount Cook resident feared that could then be used by the National Party as canvassing information, to help it get voters out on election day, or to cross reference party allegiance with policy preferences, effectively acting as market research for the party.

Labour deputy leader Grant Robertson, the MP for Wellington Central, said the questionnaire was ''unusual'' but Parliamentary officials were thorough in checking material, so it was almost certainly within the rules.

''It's unusual, particularly the fact the form has the person's name on it - so it can't be anonymous.''

He said it potentially exposed a loophole and Labour may seek a review of the rules. ''But we don't make the rules.''

A spokeswoman for Key said: ''The direct mail was pre-approved for a parliamentary purpose by the Parliamentary Service. We do not disclose the costs of our direct mailouts from the leader's budget.''

The survey included a ''privacy notice'' in fine print at the end saying supplying the information was voluntary and that it was being collected ''by National MPs in their capacity as Members of Parliament to be used for electorate and related activity".

Asked if the information would be passed to the National Party for canvassing purposes, the spokeswoman said: "The purpose of the surveys is so MPs can use the information gathered to find out what policy issues constituents are interested in so they can better communicate with them. That communication can take a number of forms, including direct mail."

- © Fairfax NZ News