Johnstone, who played three tests in 2005, had already told his family and close friends before publicly coming out on current affairs programme Seven Sharp.
"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," Johnstone said.
"If I open up that door and magically make that closet disappear, then we're going to help a lot of people... "
"I pushed that side of me down deeper and deeper. I went to some interesting places."