That was in 1996, when he was elected to represent Christchurch Central.
"I was labelled by the Evening Post as Parliament's gay pom - as though every institution needed one," he said.
"Now I'm one of a good half dozen rainbow MPs here."
He said being gay or lesbian would never be irrelevant.
"The journeys of self-discovery, of public advancement which we travel, give us special insight and strength.
"The venom poured on us by more than a few in New Zealand and the great majority in some countries shows we're different, and that when some equate difference with threat then discrimination and oppression follow."
Mr Barnett said he had enjoyed 12 intensively busy years in Parliament.
And he had worked out that he had flown between Christchurch and Wellington more than 1800 times and held more than 10,400 case files for his constituents.
Mr Barnett is Labour's senior whip - which he said was like sweeping up after elephants - and he listed among his achievements his sponsorship of the Prostitution Reform Act and the part he played in promoting the Civil Union Act.
He also mentioned the media, but not in the same way some of his departing colleagues did.
"I've long ascribed to the advice that if you're going to invite a tiger to lunch, there's little point in pouring the sauce over yourself."
Mr Barnett said he might work for the United Nations and then return to New Zealand.
"We all know wisdom is all around us if only we would take the trouble to learn.
"I've grasped the opportunity, learned to my capacity, and now I move on wiser, seeking new horizons."