Prime Minister John Key's good bloke routine has worked for him for seven years straight, helping foster the image of him as a good egg, a bit of a joker, a good sport.

Key has made his informal interview slots on commercial radio a major part of his media strategy. Politically, it is is a good tactic. Commercial radio reaches people who don't give a poop about the more serious side of politics. Key knows that and plays to his audience accordingly.

The problem is his audience inevitably ends up being a lot wider than the one he originally plays to. It gets on social media and news sites.

It is usually harmless fun, however much his opponents might bridle at it. Some of us have even egged him on in such antics.

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But he has a startling tendency to go overboard. He can not even blame the hosts who come up with the ludicrous things they get him to do. He's been in the game seven years and has the choice of saying "no".

His end-of-year slots this year were a case in point. The first was only embarrassing, mainly for him. Confronted at The Edge by a lineup of women with ponytails and told he had a choice of pulling a ponytail or singing All I Want for Christmas, the only real option was the latter. The second on The Rock was more marginal. Asked to get into a cage with comedian Tom Furniss, the danger signs should have been flashing.

READ MORE: John Key caught up in a rock radio stunt

Just as Caesar thrice refused the crown, Key resisted requests to get into the cage three times. But then he caved. Once in the cage, the 'drop the soap' jokes came out and Key played along, analysing the soap.

Key may have been oblivious to the prison rape references and crude Deliverance quote involved. But no Prime Minister should put himself in a cage, let alone one that ends up with jokes about prison rape.

Politicians tend to grow into the role of Prime Minister and gradually don the gravitas involved. Key is no exception. He has gone from being called "John" to being called "Prime Minister". Over his seven years, he has gained in status on the international stage and become something of a statesman.

That is one reason he can no longer get away with things he was excused for in his early years.

Three weeks ago, Key was being feted at Apec in a casual conversation between US President Barack Obama and Australian Prime Minister Malcolm Turnbull.

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Two weeks ago he was meeting other Commonwealth leaders at Chogm and laying a wreath for the terror attacks in Paris before historic climate change talks.

Little wonder it was jarring to see him sitting in a cage playing along while a radio host makes prison rape jokes about soap.