The Electoral Commission warned RadioLive to act with "a high degree of caution" if it went ahead with its Prime Minister's Hour hosted by John Key, saying it risked running foul of the law because the election was so close.
The commission also advised that the broadcaster risked serious penalties if the live programme breached those rules - an issue it is now considering after Labour laid a complaint that it had breached both the Broadcasting Act and Electoral Act.
Mr Key hosted the hour-long show on September 30. He declared it an "election-free zone" before he interviewed guests including Sir Peter Jackson, Richie McCaw and Sir Richard Branson.
The commission warned RadioLive beforehand that it had to act with extreme care because of the closeness of the election and because strict pre-election broadcasting rules had come into effect.
It said the station had to ensure the programme neither encouraged nor appeared to encourage voters to vote for National or Mr Key.