He voted against decriminalising male homosexuality in 1986.
At its final reading in Parliament, he described the Homosexual Law Reform Bill as "evil".
"This day will be remembered as a sad and sickening day for New Zealand," he said.
Labour MP Phil Twyford also confirmed he is voting for the bill.
In a post on his Facebook page, Mr Twyford said voting for the bill was "the right thing to do".
He said his decision to support marriage equality came after weeks of discussions with people in his Te Atatu electorate.
"I have been heartened to hear so many people express the view that all New Zealanders should have every chance in life regardless of colour, sex, where they grew up, what school they went to, or who they choose to love."
- Herald Online