"In the end, very few people come to Parliament with a background of perfection. Some do, but most people have made the odd mistake in their life. The big challenge is learning from that mistake, he has learnt from that mistake."
Mr Key said he would be "absolutely amazed" if there weren't other MPs in Parliament with previous convictions.
He said as part of his role overseeing the Honours List, he looked in to the previous convictions of some of the nominees, who would often have convictions for driving offences.
"If that's what's happening with some of the finest New Zealanders we're recommending for honours, I'd find it pretty difficult to believe that 121 MPs who currently sit in our Parliament don't have criminal convictions."
Mr Key said he had never had a criminal conviction.
"I'm not saying I've lived a life of blameless excellence. I once got a speeding ticket in Australia ... I was about 7km over and yes I paid it. Schoolboy error."