The Electoral Commission is looking into an ad placed by the horse racing stalwart Sir Patrick Hogan during last year's election campaign.
The ad, in the racing industry publication The Informant last September, urged people to party vote New Zealand First because of its leader Winston Peters' support for the racing industry.
The Electoral Act requires people who take out ads promoting a political party during an election campaign to have the party's permission.
New Zealand First said its party secretary did not authorise any third party advertisements.
Otago University public law professor Andrew Geddis said if Sir Patrick did not have the necessary authorisation he may have committed an illegal practice and could be fined up to $10,000.
A spokesperson for the Electoral Commission said it had not received any complaints about the ad but would look into the matter.
Sir Patrick has not yet responded to a request for comment.