National Party MP Tim Macindoe and a Hamilton radio station have been referred to the police, accused of broadcasting an election advertisement outside the proper period.
Under the Broadcasting Act, election programmes can be aired only between 20 August and 19 September.
The Electoral Commission contacted the police about Mr Macindoe and the community radio station, Free FM, on Tuesday.
The commission believes the advertisement is in breach of the Broadcasting Act, an offence carrying a fine of up to $100,000.
It says the ad also did not have a promoter statement, as required under the Electoral Act.