Marriage is for heterosexuals only, though new legislation aims to ensure no couples are disadvantaged under the law, Prime Minister Helen Clark said today.

MPs will this week vote on that controversial legislation -- the Civil Union Bill and a companion bill, together dubbed the gay marriage bill by critics.

The long-delayed bill creates a new form of legal relationship -- a registered civil union -- that would apply to same-sex relationships and heterosexual couples.

Helen Clark told Newstalk ZB questions as to whether the legislation would give gay couples the same status as heterosexual married couples were "really not that relevant".

"Nobody should be disadvantaged," she said.

"Should people who want to have legal recognition of a marriage be able to get it? The Government says yes, but you can't marry. Marriage is only for heterosexuals.

"The Government is not -- underline -- not, changing the Marriage Act. That will remain as an option only for heterosexual couples."

A civil union would run in parallel to marriage and the same rules would apply, journalists were told in a briefing last week.

The only difference between a civil union and marriage was the name and that same-sex couples could not get married.

The bill will be tabled in Parliament today. MPs will cast a conscience vote on Wednesday or Thursday.

Running alongside the Civil Union Bill will be the Relationships (Statutory References) Bill that removes discrimination based on marital status.

It amends over 1000 provisions across more than 100 laws covering commerce, education, health, taxation and social assistance.

The companion bill aims to take out discrimination which disadvantages married couples in some cases and unmarried couples, whether gay or heterosexual, in others, Helen Clark said.

"We're trying to even it up," she said.

Associate Justice Minister David Benson-Pope said while there was greater support for the "nuts and bolts" statutory references bill, he was confident both bills would have enough support to be sent to a select committee.

Much of the opposition to the bill had been bred by ignorance about its contents and scaremongering about changes to the institution of marriage, he said.

Once the bill saw the light of day he believed support would grow.

The Labour caucus has been divided over the bill, with even some of its supporters suggesting the controversy was not worth it.

On the other side of the political spectrum, National is also divided on the issue, with MPs set to cast their conscience votes on both sides.

Indications are that a "good number" oppose it, whip John Carter told NZPA today.

National leader Don Brash has said he has no personal problem with the proposals as they do not undermine marriage.

But his families spokeswoman, Judith Collins, has been very critical, accusing Labour of being cynical in its attitude to marriage.

She attacked Mr Benson-Pope for saying many politicians married only to remove barriers and were put off by the religious associations that come with marriage.

Under the proposals, couples between 16 and 18 will have to get their parents' permission to enter a civil union and as a result the maximum age where parental consent is required for marriage also falls from 20 to 18.

Some parties are less split over the issue. All United Future MPs are expected to vote against the bill and all Greens in favour.

The last conscience issue in Parliament was the prostitution law reform bill that passed by the narrowest of margins after one MP who was against the bill decided to abstain at the last moment.

That bill was marked by fierce lobbying from both sides of the political and social spectrum. A similar process for the Civil Union Bill has been under way for some time.