"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.