Prime Minister John Key coming out saying he isn't personally opposed to same-sex marriage has prompted a lobby group to question the state's right to re-invent marriage.

Mr Key was asked about the issue after US president Barack Obama said he supports same-sex marriage, ending a period of equivocation on a divisive issue.

"I am not personally opposed to gay marriage," Mr Key said.

"It's possible that parliament may consider a member's bill at some stage, but it is not on the government's agenda."

Mr Key's position has triggered a response from lobby group Family First, which says the state didn't invent marriage and has no authority to re-invent it.

"A homosexual cannot now legally marry. But neither can a whole lot of other people," the group's national director, Bob McCoskrie, said.

"A five-year-old boy cannot marry. Three people cannot get married to each other. A married man can't marry another person. Two old aunties living together cannot marry. A father cannot marry his adult daughter. A football team cannot enact group marriage - the list is endless."

Being pro-marriage and wanting to maintain its definition as being between a man and a woman was not "anti-gay", Mr McCoskrie said.

Mr Key earlier said he didn't think there was any clamour for gay marriage in New Zealand.

Labour leader David Shearer says he personally supported marriage equality.