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.
"The Prime Minister hosting a show of this nature is likely to attract close scrutiny at this time in the electoral cycle. If RadioLive was to proceed with the broadcast, it would need to be confident it [had] adequate controls to ensure that the programme did not breach the election advertising rules."
RadioLive has denied the show was election-related. It sought the advice on September 28 - two days before the programme went to air - and the advice was provided on September 29.
MediaWorks' general manager of talk brands, Jana Rangooni, said the station's advice was that it had not breached the rules and had retained editorial control.
"We carefully thought this through and took advice to ensure that the PM's Hour was neither election programming nor election advertising."
Labour has argued it was valuable self-promotion for Mr Key and unfair to other leaders.
The email correspondence released also shows Labour leader Phil Goff's press secretary Fran Mold raised concerns with RadioLive the day before the broadcast. She had emailed RadioLive seeking a similar opportunity for Mr Goff, saying The Prime Minister's Hour was unfair to other leaders so soon before the election.
Ms Rangooni refused the request, saying the programme was "nothing to do with the election" but aimed at enhancing RadioLive's brand.
Ms Mold objected that that was a "naive" attitude, especially given the advertising of the programme.