New Zealand Prime Minister John Key has come under fire for being part of a radio stunt that jokingly alluded to sexual assault in prison.

The PM was live on local radio station The Rock FM when one of the hosts, Tom Furniss, asked Mr Key to step inside a cage.

At first Mr Key was reluctant, but eventually gave into the request after heckling from the other hosts.

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While the pair were in the cage, Mr Furniss dropped a bar of soap and requested the PM picked it up – which was met with laughter.

Mr Key proceeded to pick up the soap, when Mr Furniss said: “You’ve got a pretty little mouth, Prime Minister.”

The other radio hosts roared with laughter and then Mr Key said the soap “smelt bad and was greasy”, wondering where it had come from.

A spokesperson for Mr Key said the interviews were meant to be light hearted. Photo: The Rock FM

Another radio host said it had been taken from the men’s urinal.

The term ‘dropping the soap’ refers to prison rape.

The backlash came quickly online, with Auckland psychotherapist Kyle MacDonald creating an petition to have the PM stripped of a White Ribbon ambassador position –  so far it’s received more than 6,000 signatures.

“I have become increasingly concerned about the actions of our Prime Minister, and how they show a complete lack of understanding about the impacts of sexual and physical violence in this country,” Mr MacDonald said about Key’s comments.

“His ‘prison rape’ joke on radio yesterday was the last straw.

“I believe it is no longer OK [sic] for John Key to make light of sexual abuse and sexual violence, and so I ask you to join me in calling on the White Ribbon organization [sic] to remove John Key as an ambassador.”

This comes as female NZ MPs were asked to leave Parliament in November after declarations saying they has been sexual assault victims were ruled “out of order” by he Speaker, The Guardian reported.

Their actions came after Mr Key that same week said Labor and Green MPs were “backing the rapists” in a disagreement about the detention of New Zealanders by the Australian government.

A spokesperson for Mr Key told The Guardian that: “The Prime Minister does these interviews in the spirit of Christmas and the content is decided by the hosts.

“The interviews are meant to be light hearted, and the Prime Minister hopes the media and the public take them that way.”

Meanwhile, Deborah Russell, a feminist commentator and Massey University lecturer, aired her disgust on Twitter.

The PM playing along with rape jokes makes it very clear how little regard he has for changing the law around sexual violence.

— Deborah Russell (@BeeFaerie) December 16, 2015

Watch the video here: