Church ministers with a moral objection to gay marriage would be criminalised by refusing to wed same-sex couples if a new bill becomes law, a legal opinion states.

* Marriage equality: full coverage

The view of Ian Bassett on Louisa Wall's ''marriage equality'' bill, commissioned by conservative lobby group Family First, was released today. It came as a petition against the bill signed by about 50,000 people was delivered to Parliament this afternoon.

The bill is expected to pass its first reading when it is debated at Parliament tomorrow.

Bassett's opinion suggests church ministers, marriage celebrants and even wedding photographers who withheld their services to same-sex couples on the grounds of a moral objection to gay marriage would be breaking the law if Wall's bill passed.

It rejects the view of the Human Rights Commission, issued late last week, which said religious ministers would still be allowed to refuse to marry anyone - including same-sex couples - if Wall's bill passed.

The Human Rights Commission statement was ''legally incorrect,'' Bassett said.

"If a marriage celebrant is available to exercise his or her statutory role, he or she cannot refuse to do so by reason of any prohibited ground of discrimination (ie. such as sexual orientation),'' his opinion stated.

The practical effect of the bill, if enacted, would be that church ministers with moral objections to same sex marriage would likely "withdraw totally from the statutory role of marriage celebrant; withdraw totally from providing religious marriage ceremonies to the public; continue providing religious marriage ceremonies only to members of his or her own church''.

Also as a result of the bill, a church could not refuse to rent out its premises to its members for a same-sex wedding on the grounds of their sexuality.

Church ministers with moral objections to renting out church facilities for same sex marriage functions and who wanted to ''avoid the risk of being forced to do so,'' would ''likely withdraw from making their church facilities available to any member of the public, including play-groups, senior citizen or other community groups''.

''The consequences for churches and communities would be significant,'' Bassett's opinion said.

However, Wall said pastors and church ministers would not be obliged to marry anyone if her bill passed.

"They will retain all the rights that they currently have. They choose who they marry, they choose what definition of marriage they have and so my bill isn't going to affect them in any way,'' Wall said.

"We should have an open and honest debate. What I don't like is scaremongering and the fact that ministers are saying that they will have to go to jail. That's not right in our country and it won't happen. I will protect the rights of our ministers to define marriage as they see fit."

Wall said she believed there was "a solid 60" of the necessary votes to pass her bill and "a few of the undecided will hopefully join our team".

Family First director Bob McCoskrie said MPs should reject Wall's bill and instead ''focus on more urgent issues''.

"If marriage is redefined once, there is nothing to stop it continuing to be redefined to allow polygamy, polyamory and adult incest relationships,'' McCoskrie said.

''Throughout history and in virtually all human societies, marriage has always been the union of a man and a woman.''

Bassett's opinion said the assurances of the Human Rights Commission of any MP were of "no legal effect" unless an exemption was provided for in the legislation.

Bassett has previously represented the anti-abortion group Right to Life in some of its legal fights.

- © Fairfax NZ News