''To show other people who might be struggling with their sexuality, not only that it's OK to be gay, but it's a good thing, and it won't change who you are or limit what you can achieve. At the end of the day, it's only one of the many things that define me as a person."
Manson said he told friends two and a half years ago after being named in the Olympic team and had now decided to go further.
"I feel like there has always been a confident, outgoing side of me dying to come out. But from an early age I realized that I was different from most other guys.
''I desperately tried to hide that 'different' side of myself. In doing so I inadvertently became very quiet and shy, shutting myself off and avoiding attention for fear that someone might discover my deepest, darkest secret: I'm gay," he wrote for outsports.com.
As his rowing career progressed he was terrified of anyone finding out he was gay, especially his team-mates.
"I seriously thought that if anyone found out I wouldn't be able to row anymore. The thought of coming out, in my mind, felt so limiting and terrifying."
When he did tell friends, their reaction helped ease his anguish.
"It was the night after being named to the NZ rowing team for the London Olympics that I came out to more people. I was ecstatic to be selected in the quad. Going to the Olympics was a dream of mine ever since watching the Sydney Games when I was 10-years-old.
''We were having a few drinks and near the end of the night I had a heart-to-heart with two of my friends. In a very emotional state I told them that I was different, and then finally that I was gay.
"Slowly I told more friends over the next few months, and by the end of the Olympics everyone knew, including the rest of my crew.
"I'm sure word spread a lot faster than I told people, and for a while it was a bit of a guessing game of who knows and who doesn't.
"Much to my surprise, everyone was fine with it. I didn't have a single bad reaction, and most people were demonstrably supportive."
Manson teamed with Joseph Sullivan to win the national double sculls titles in 2012 and 2013.
Last year he teamed with Michael Arms in the men's double scull for international events and they dominated the World Cup events, winning gold in Sydney, Eton and Lucerne.