Progressive leader Jim Anderton has been referred to the police for possible breaches of the Electoral Act.
Right-wing blogger Whaleoil laid a complaint with the Electoral Commission last month about a letter that Mr Anderton had sent to voters in his Wigram electorate endorsing the Labour candidate Megan Woods.
In the letter, Mr Anderton wrote, "I urge you to give Megan your vote".
The letter was authorised by Mr Anderton.
Chief electoral officer Robert Peden, in a letter sent to WhaleOil and published on the blog, said that the matter had been referred to the police.