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.
"It is the commission's view that the publication of the letter constitutes a breach of section 204B of the Electoral Act because Mr Anderton is involved in the administration of the affairs of the Progressive Party and is therefore not entitled to promote an election advertisement," the letter said.
Under the Electoral Act, a person can be a promoter of election advertising if they are a party secretary, a candidate, a registered promoter or an unregistered promoter.
A person cannot be an unregistered promoter if they are involved in the administration of the affairs of a political party.
Mr Anderton could face a fine of up to $40,000, if he is deemed to be an unregistered promoter, or $10,000 if not.
Earlier this month a Facebook advertisement by Megan Woods was also referred to police for possibly breaching the Electoral Act, as was a newspaper advertisement in Dunedin's The Star by National MP Michael Woodhouse.
Mr Woodhouse, who is standing in Dunedin North, admitted the oversight after a monthly editorial in the community paper failed to include a promoter statement.
In August, Labour MP Charles Chauvel was referred to police for failing to include promoter statements on three advertisements.