The gay marriage bill is not just dividing Parliament, it has also sparked a public - but polite - disagreement inside one MP's family.

The daughter of one of the bill's opponents, West Coast Tasman MP Damien O'Connor, has taken to Facebook to urge him to change his mind and back Labour MP Louisa Wall's member's bill with his conscience vote.

On her father's Facebook page, Bridgette O'Connor said people did not choose to be gay.

"I hope he does vote for gay marriage as society needs to wake up and realise these are normal people who deserve the same treatment and rights as everyone else," she said.

"It is who they are. They cannot change this and are people like you and I. Why should they be denied the same human right everyone else is entitled to just because they love someone the same sex?"

Mr O'Connor posted in reply that he was proud of his daughter, "who thinks for herself and has a strong sense of social justice".

"But sometimes we disagree. And so it will be throughout New Zealand. We must respect one another's opinions but ignore the extremes from both sides of the debate," he said.

Meanwhile, Mangere MP Su'a William Sio has been rapped over the knuckles for his comments that the gay marriage bill could cost Labour the next election.

Labour sources said it had been made clear to him that Labour MPs were free to express their personal views, but commenting on its impact on the Labour Party was completely outside the agreed "rules of engagement".

Mr Sio yesterday said he had not talked to the leadership about it. "I will not be doing any more statements about that."

Labour's Pacific sector vice-president Efu Koka, who has backed Mr Sio's opposition to the bill, said Ms Wall had issued a press statement before consulting the party. There was discussion before it was put in as a bill and some people had expressed opposition. "Yes. I'm one of them. I said we don't really need this."

A Labour spokeswoman said gay marriage was party policy at the last election.

Prime Minister John Key yesterday confirmed he would support the bill.

- © Fairfax NZ News