Bill English has already pulled a U-turn before being signed in as Prime Minister - saying he now supports marriage equality.
"I'd probably vote differently now on the gay marriage issue, I don't think that gay marriage is a threat to anyone else's marriage," he said.
"Just seeing the impact it's had for couples and the fact that it doesn't actually erode marriage. In some ways it's an affirmation of the concept."
He voted against the Civil Union Bill in 2004 as well as the Marriage (Definition of Marriage) Amendment Bill in 2013.
The soon-to-be Prime Minister is "an active Catholic and proud of it".
"It's been an influence in forming my judgement forming my conscience. It's no guarantee of virtue or perfection, that's for sure," he said.
"It doesn't define me but it's an important influence."
But he accepts his faith might put him out of step with other New Zealanders on issues like abortion and euthanasia.
"My views around abortion and euthanasia I think are pretty well-known," he said.
Mr English says he would vote against a bill allowing assisted dying - but he would let his MPs decide for themselves in a conscience vote.