"Three years ago I remember standing on a podium with you singing your party's praises as you announced that you were spending $1.9b on mental health," he writes.
"As long as I live I will never forget that day. There was such a euphoric feeling in the air, full of optimism and hope and I believed with all my heart things were about to change, finally we had a government who cared."
But three years on, he says he feels they've both "let everybody down".
"I know none of this is your fault Prime Minister and I know you truly care about our children but the system is broken and it seems to our most vulnerable Kiwis and their families that no one is trying to fix it," he says.