Prime Minister John Key says euthanasia already happens in our hospitals - and if he was terminally ill, he would consider it.

Doctors disagreed with him last night, saying his view of the situation was too simplistic.

Mr Key said yesterday that he could understand the argument that legalising euthanasia might put pressure on the elderly to end their lives early, in the face of "rapacious grandkids", but "I don't really buy that argument".

"I think there's a lot of euthanasia that effectively happens in our hospitals," he told Newstalk ZB.

". . . If I had terminal cancer, I had a few weeks to live, I was in tremendous amount of pain - if they just effectively wanted to turn off the switch and legalise that by legalising euthanasia, I'd want that."

Labour MP Maryan Street is drafting a member's bill to legalise euthanasia - and the emotive subject is already sparking debate even before it is in the ballot.

Mr Key signalled his broad support for euthanasia, but said he was not sure if he could support Ms Street's bill, which looked likely to have some "quirks" that he did not agree with.

"So I may or may not vote for that bill, [but] the broader principle I support."

In 2003, he supported a bill promoted by NZ First MP Peter Brown legalising euthanasia. It was defeated on its first vote.

Ian Powell, director of the Association of Salaried Medical Specialists, which represents senior doctors, said he did not think euthanasia was happening in hospitals the way Mr Key made it out to be.

"The situation is much more complex than that . . . Sometimes continuing a treatment can prolong the agony for a patient, and not even keep the patient alive.

"By not prolonging the agony . . . even though the intent is not for the patient to die, it is sometimes a consequence."

Also yesterday, Mr Key revealed that his views on gay marriage had changed, largely in response to having children.

He has stated his support for a bill legalising gay marriage, but said: "If you asked me questions on same-sex marriage 20 or 30 years ago, I might have taken a different view.

"But the way I look at the world now is I look at our own kids and my No 1 priority for our children is to be happy, healthy and safe.

"I don't think they are gay, but if they came along and said they were, I wouldn't love them any less."

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