By SABC Sport
31st January 2023
The 43-year-old, who played three Tests for New Zealand in 2005, said he had been "leading a double life", but hopes his decision will put an end to "the pressure and stigma surrounding the issue".
Johnstone said he told his family and friends "a long time ago" before coming out publicly on New Zealand's Steven Sharp current affairs programme.
"Within myself, I was never comfortable with the whole concept and my dream was to be an All Black," said the former prop, who played for Canterbury and Biarritz, and also spent a brief spell at the Ospreys.
"My view of an All Black was manly, strong, possibly with a wife and kids."
He continued: "I pushed that side of me down deeper and deeper and I have been to some interesting places with that.
"It would come to the surface and I may have a bad game and I would look at that side of me and blame that side. But it slowly starts to affect you. It is hard living a double life or living a lie.
"If I can be the first All Black that comes out as gay and take away the pressure and stigma surrounding the issue, it can actually help other people.
"Then the public will know that there is one in amongst the All Blacks and it could be one of the final pieces in the puzzle for New Zealand sports-wise that gives everyone closure."
Following Johnstone's announcement, New Zealand Rugby CEO Mark Robinson said in a statement: "Your strength and visibility will pave the way for others in our game.
"Rugby is a sport that is welcoming to everyone and a place where people should feel safe to be who they are.
"We know that there are people who have not always been comfortable to be who they are in rugby. We want to be clear, no matter who you love, rugby has your back."