Men needed to change their lifestyles and mindsets before they do long-lasting damage to their children, said Mr Tamati, but it's a process that takes time.
"If you've been 38 years a perpetrator, 38 weeks is not going to make a difference on a stopping violence programme."
Mr Tamati was a victim of violence for much of his life, so when he had his own family it was a normal way of dealing with issues.
In 1992, he bashed up his youngest daughter with a platform shoe and soon after his wife left with the kids. His first intuition was to use violence to try to keep them.
It was when his daughter told him he should get help that he finally did something about it.
The stopping-violence courses had a reputation for being "poofters' clubs" within his circle of friends, so he signed up without telling anyone. He had to go through the 20-week course twice to start feeling under control, and said it was an ongoing battle.
"I'm still on the journey, it's a journey through hell.
"To try and come through it unscathed is impossible, and part of that has been my own demons," Mr Tamati said.
He describes his work as an It's Not OK campaigner as penance: Telling his story to audiences around the country brings other men forward to tell theirs, which makes him feel not so bad.
His six kids, often through their parenting of his nine grandchildren, help, too.
Mr Tamati said learning about the "naughty step" was a revelation in child discipline.
"If that's what I've got to know and learn so I don't have to go dark places again, that's okay."
