United Future leader Peter Dunne has labelled his former MP Gordon Copeland "a serial nutter" over his comments likening the concept of gay marriage to apartheid-era treatment of Maori as "honorary whites".
Mr Copeland, who broke away from United Future in 2007 and left parliament a year later, made a submission against the Marriage Amendment Bill to parliament's government administration select committee on Wednesday.
He said because gay and straight couples are inherently "different", the word "marriage" should not be applied to same-sex unions, adding that the issue reminds him of the South African government's suggestion that Maori rugby players could accompany the All Blacks as "honorary whites" during apartheid.
"That suggestion was repugnant to New Zealanders because it debased Maori in pursuit of a thoroughly unreasonable political agenda," Mr Copeland said.
"In proposing that same-sex unions be called `marriage', when they're clearly not marital, this bill debases marriage which is our precious and our most important institution in pursuit, again, of a thoroughly unreasonable political agenda."
Mr Dunne quickly distanced his party from Mr Copeland's remarks, calling him "a serial nutter".
He said he was "delighted" when Mr Copeland, now understood to be a Conservative Party member, left his party.
Labour MP Charles Chauvel said the apartheid comment was "unfortunate".
"When [opponents of the bill] say `look, straight people are just fundamentally different from gay people' and `gay people should be satisfied with civil unions because separate but equal is fine', what they're doing - whether they realise or not - is adopting the language of apartheid," he told media.
"There's no doubt that a lot of the opposition to marriage quality comes from people who just don't like the idea of people being different and there's nothing that can be done to change that."
Mr Copeland also told the select committee he believes civil unions are sufficient for gay couples.
"We've all seen on television the people having their champagne and so forth at civil unions. So it seems to me that this move - from my point of view - is actually to try and pretend that that's not good enough."