Prime Minister John Key's latest live radio appearances - which included jokes about prison rape and the mangling of a Mariah Carey Christmas tune - might not turn off loyal voters, but will hurt his long-term reputation, says a political analyst.

Photo: AFP

While doing a round of media interviews with music radio stations yesterday, Mr Key joined in with the DJs on The Rock who were making jokes about prison rape.

Male Survivors of Sexual Assault Trust head Ken Clearwater said no one would make a joke like that if it involved female rape. Earlier this year, US President Barack Obama condemned prison rape jokes, saying they were unacceptable and jokes should not be made about it in popular culture.

It was not the PM's only on-air gaffe yesterday, as he sang an off-key rendition of All I Want for Christmas, which can be seen here, during a visit to The Edge.

Labour Party deputy leader Annette King said the participation in the radio stunt was demeaning to the role of Prime Minister.

Ms King said Mr Key had gone too far and people would be concerned he was prepared to do anything for publicity.

"Everybody likes a bit of fun but we also want to look up to and have our Prime Minister respected. This sort of behaviour, along with quite a number of things over the last couple of years, really makes us the laughing stock."

She wanted to be charitable, she said, so would assume Mr Key did not know the connotations of picking up a bar of soap.

New Zealand First leader Winston Peters said he did not consider the radio prank very funny.

"I'd have to say that [if] the Prime Minister wants to make a prize dickhead of himself on some pirate radio, that's over to the listeners to decide and not for me. Surely, the public can decide for themselves what they think of that."

Mr Peters said the stunt probably would not affect Mr Key's popularity but that was not the point.

Political commentator Bryce Edwards said the Prime Minister's "jokey-blokey" persona was still popular with many New Zealanders and he could not imagine that many listeners of The Rock would take offence.

He told Morning Report today that Mr Key did cultivate this image and it had been a successful formula during his tenure.

"I think we've seen this year that it has been eroded, or that there are more questions about it, but he remains a very popular prime minister and it's because he is not aloof like a typical prime minister. He's not a stick in the mud, he does go in for these jokes.

"And it's part of a media strategy that is quite successful - he avoids things like Morning Report and hard news - he goes for the more soft news. It's a good way of getting out to people who aren't really engaged in politics, and that's his way of keeping in people's minds, but there is always a fine line."

Dr Edwards said there might be little short-term effect, but the kinds of appearances he has made this week could hurt his long-term reputation.

"He won't go down in history as much of a stately person as he might want to be. I don't think it's going to hurt him amongst that core middle-class New Zealand swing voter, but his reputation is just a wee bit more tarnished as a result."

He said the success of Mr Key's strategy showed he still often had a good political radar, but not always when it came to sensitive social issues and the way he handled himself personally.

"He just hasn't really realised that things have changed, the whole public atmosphere about sexual issues has really changed in the past 10 years. And I think he might have got away with it 10 years ago, but it's not really acceptable for a lot of New Zealanders now."

A spokesperson for the Prime Minister's office said in a statement that Mr Key did such interviews in the spirit of Christmas, and the content was decided by the radio hosts.

She said Mr Key hoped the media and the public treated the interview as light-hearted.

I've been a National Party member for 25 years. I'm gay. And a sexual assault survivor. No, Prime Minister. Rape isn't a joke. @johnkeypm, — Craig Ranapia (@CMRanapia) December 16, 2015

On Monday, the Law Commission releases a major review into the treatment of sexual violence cases. Two days later, the PM makes a rape joke. — Deborah Russell (@BeeFaerie) December 16, 2015

I think we should all be calling on @WhiteRibbonNZ to dump @johnkeypm as an ambassador. Don't you? #nzpol — Kyle MacDonald (@kylemacd) December 16, 2015