As a 14-year-old, Grant Robertson sat in his family's Dunedin home and carefully cut out the names of MPs who had voted to legalise sex between men.

They included current MPs Annette King, Phil Goff, Trevor Mallard and Peter Dunne, whose support meant the Homosexual Law Reform Act narrowly passed.

Saturday marks 30 years since that milestone in the gay rights movement, an anniversary that will be celebrated in Parliament tonight.

Mr Robertson, now Labour's finance spokesman, whose family were active members of the Presbyterian Church, remembers the intense debate that swept New Zealand.

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"The petition organised to oppose homosexual law reform circulated through the churches, and I vividly remember it showing up at a church youth group picnic.

"I was coming to terms with being gay. And even for me it was quite a threatening environment. And certainly when I think about the older men and women who during that campaign had to put their own lives out there, at a time when it was still a crime - it was an enormously threatening environment for those people."

Mr Robertson believed his late father signed the petition against a law change.

"Good people got carried along. Not because they were full of hate, but because they were full of fear."

The act was introduced by Wellington Central Labour MP Fran Wilde. It decriminalised sexual relations between men aged 16 and over.

Three National MPs voted for the law change. A second part of the legislation provided protection from discrimination, but didn't progress.

Green Party MP Kevin Hague, then 26 and involved in the campaign to change the law, listened on the radio with his partner, Ian, as the law passed 49 to 44 votes.

Mr Hague said a strategy in the lead-up to the vote was to show that gay people weren't to be feared, and were people's friends, family and colleagues.

That required men to come out publicly - no easy task amidst the heat of debate, when some campaigners were beaten up.

Full-page adverts in the Herald ran, Mr Hague said, with small type featuring the names of hundreds of men who had agreed to be identified as gay.

"For many people that was their act of coming out. You can imagine those sort of tea room conversations - of people poring through the list to see whose names they recognise."

Today, Mr Hague will table a petition in Parliament calling for an official apology to men convicted before 1986, and for their convictions to be quashed.

Tonight's anniversary event is co-hosted by Labour MP Louisa Wall, who successfully introduced marriage equality legislation in 2013, and National MP and Attorney-General Christopher Finlayson.

Some within Labour had advised Mr Dunne not to vote for reform, because it could lead to a voter backlash in the marginal seat of Ohariu. Labour deputy leader Annette King, then a first-term MP representing Horowhenua, ignored similar counsel. Both retained their seats.

Mr Dunne said the mood was extraordinary. "It's hard to imagine now - people filling Parliament grounds with protests, shouting obscenities ... it was homophobic in the obscene. I don't think we have seen anything comparable since then."