He said modern medical practice was to give terminally ill patients pain relief and allow the natural process of death to occur.
"The palliative care would not do anything to prolong their life or to shorten their life. What I would say is in that scenario I ... could understand the speeding up of the process," he said. "The bill goes a lot further than that. In the situation where grandma is 92 [and people just want her to go], that's not acceptable."
Mr Key voted for a bill brought by former NZ First MP Peter Brown in 2003 that would have allowed people with terminal illnesses to seek medical assistance. That bill was defeated by 60 votes to 57. He said Mr Brown introduced the bill because he saw the pain his wife went through before she died of cancer.
Labour leader David Cunliffe declined to comment on the issue yesterday and Ms Street did not return calls.
Mr McCoskrie also asked Mr Key about abortion, gay marriage, smacking, the drinking age and decriminalising marijuana. Mr Key did not support changing the law on any of those issues.
However, answering another question on prostitution, he said Customs Minister Nicky Wagner was working on a policy to tackle street prostitution in residential areas.
"The whole argument around prostitution law reform that was put up was that it would eliminate street prostitution and there would be a safe environment where girls could work," he said.
"That hasn't worked. If you go out to South Auckland, there's 14-year-old girls out there. Everyone is struggling to work out exactly how to make it work."