Johnstone made the announcement on a New Zealand current affairs television program on Monday, saying he had been "living a lie" and leading "a double life" before acknowledging his sexuality.
"If I open up that door and magically make that closet disappear, then we're going to help a lot of people," said Johnstone, who also played 38 Super Rugby matches for the Christchurch-based Crusaders.
"I pushed that side of me down deeper and deeper. I went to some interesting places."
New Zealand Rugby quickly supported Johnstone's decision to come out, praising him for "having the courage to share his story and helping create a more inclusive game".
NZR chief executive Mark Robinson said Johnstone's "strength and visibility will pave the way for others".
"Rugby is a sport that is welcoming to everyone and a place where people should feel safe to be who they are. We want to be clear, no matter who you love rugby has your back," he said in a statement.
New Zealand Sports Minister and former deputy Prime Minister Grant Robertson, who is gay, said Johnstone's announcement was a "big moment" and thanked him for "blazing this trail".
"Now with the All Blacks another barrier has been broken. I hope it inspires future generations to be open, happy and comfortable. There is still a long way to go, but this feels a very significant step," he said on Instagram.
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