Jim Anderton has been referred to the police for an alleged breach of the Electoral Act.
It follows a complaint to the Electoral Commission from blogger Cameron Slater over a letter Mr Anderton sent out to voters in Wigram.
Mr Slater says the letter endorses Labour candidate Megan Woods.
He claims it breaches the Act because Mr Anderton is involved in the administration of the Progressive Party and is therefore not entitled to promote an election advertisement.
The Electoral Commission says the referral has been made for an alleged breach of section 204B of the Electoral Act.
Jim Anderton joins a rapidly-growing list of politicians and parties under investigation by the Police. We understand however that no decisions will be made on any of the cases before the election, which is far from an ideal situation.
Anderton is, of course, one of the longest-serving members of the just-dissolved Parliament, and if anyone whould have known the rules, it was him. We will await the Police decision with interest.