ANIKA MOA: The singer will perform at a free gig to be held in Christchurch during tonight's Parliamentary debate.

The dividing lines have been drawn and the final showdown takes place tonight as our politicians decide whether same-sex couples should be allowed to marry. KATE CHAPMAN looks at all sides of the debate: the good, the bad and the downright crazy.

Tattooed on television presenter Tamati Coffey's arm is a constant reminder that says: "Everything is as it's supposed to be right now". It is a timely message as New Zealand looks set to become the 13th country to legalise gay marriage.

Coffey cannot be in Parliament this evening as MPs vote on the third reading of Louisa Wall's member's bill to amend the Marriage Act.

In Christchurch a free gig is being held at the Pallet Pavilion with performers including Anika Moa, who has been in a civil union with Australian burlesque dancer Azaria Universe since 2010.

Coffey and partner Tim Smith will be watching the action from Britain, the latest leg of their OE, having a celebratory drink and maybe even doing a spot of wedding planning.

"For me, the change will mean big things. Personally, Tim and I are both keen to upgrade our civil union to a marriage certificate."

For the wider gay, lesbian, bisexual and transgender community a positive vote would mean they could plan for the good times, young people would not have to be ashamed, and they could share in jokes about the trials and triumphs of "our marriages", Coffey said.

"I feel amazing to be alive at this turning point in New Zealand's history. I'm trying to commit to memory all that I can about it, so that I can retell the stories to my grandkids."

He is proud to think he may have played a part in changing perceptions.

Wall, as the bill's sponsor, is overwhelmed by support from people both gay and straight. "If this is my enduring legacy then I will be a very happy politician."

Same-sex couples were a marginalised minority but they deserved the same rights as other citizens, she said.

She was surprised by the level of scaremongering and inaccuracies by those suggesting it was the beginning of a slippery slope toward bestiality, paedophilia and incest - acts that she points out are illegal.

Conservative lobby group Family First has been running an anti-gay marriage campaign, saying the move would undermine the family.

Since the bill was first introduced to Parliament last June, MPs from both sides of the House - gay and straight - have been bombarded with messages urging them to vote one way or another.

Some have appalled MPs on both sides of the debate. Many have had the opposite effect from their intention, driving people to support the bill in disgust.

One that arrived in an unmarked envelope claimed homosexuals were paedophiles and sexual deviants who groomed the young, vulnerable and silly.

An email yesterday said marriage should be for heterosexual people only.

"If I owned a cricket pitch in pristine condition I would not invite a rugby squad to come and play on it because they would simply destroy it," the writer said.

Another thunders: "Come together, hold assembly, you shameless 77 - before you are driven out of office, before the burning anger of the voters overtake you, for the day of reckoning will come for sure, you hollow 77, whitewashed sepulchres."

That was exactly the sort of debate Wellington Catholic Archbishop John Dew had been hoping to avoid.

For him it was important to express the church's view that marriage should be between a man and a woman for the purposes of procreation. But he wanted a robust, vigorous and respectful debate.

"We certainly don't espouse to any extremist views. From our point of view we do believe that there has been a respectful listening to each other."

However, Archbishop Dew would have liked more time for the full implications of the move to be considered because "some people would think if it's legal it's moral".

THE MOVES TO DATE

1986 Homosexual law reform which decriminalised sexual relations between men.

2001 The Netherlands becomes the first country to legalise gay marriage.

2004 Civil unions law passed here.

2005 First civil unions take place.

WHAT WILL CHANGE?

Gay couples will be able to say they're married.

As married couples they will be eligible to adopt children, usually children already being raised by those couples.

There will still be brides and grooms.

Celebrants will not be forced to marry same-sex couples if it is against their beliefs.

Churches will not have to hold same-sex couples' marriages if they do not agree with gay marriage.