The abuse wreaked emotional havoc on John, yet, in the eyes of the law, his foster mum did nothing wrong.
Police investigators told him in July - and NZ Newswire has confirmed - no law existed in the 1970s that made it illegal for women to sexually violate boys under 16.
Investigators said their hands were tied - with his foster mum technically committing no crime.
"I went cold when they told me. It was harrowing," John told NZ Newswire.
At a time when there is a focus on addressing historic sexual offending by men, particularly against Maori in state care or by Catholic priests, he is astounded female offenders can walk away scot-free.
The Ministry of Justice estimates 180,000 sexual offences were committed in 2013 and that women were three times more likely to be the victims than men.
Yet Ken Clearwater, from the Male Survivors of Sexual Abuse Trust, believes abuse against men is under-reported because they are too often seen only as perpetrators and never the victims.
He believes at least one in six boys are abused by the time they are 16 and says more than half the men coming to him for help have been abused by women.
This is supported by a 2015 report by Judge Carolyn Henwood, who chaired the government-commissioned Confidential Listening and Assistance Service, which spent seven years hearing the stories of more than 1000 boys and girls in state care before 1992.
She found as many boys were sexually abused as girls and that the abusers were both men and women in roles such as foster parents, teachers and priests among others.
For Clearwater, John's story, in which there was so much denial about female sexual offending that "there wasn't even a law for it", highlights the injustice many male victims feel.
"He is entitled to justice for what happened to him," he said.
John says his conversation with police came not only as a huge shock but was especially painful.
He had been down a long path in coming to a decision to report his foster mum.
That included numerous emotional breakdowns and battles with alcohol that put the brakes on a successful career.
He also attempted to take his own life, but was saved by hospital surgeons.
Foster placement
He believes his foster parents started with good intentions.
John had already bounced between nine foster homes when they took him in after spotting him as a 6-year-old living with an abusive family in a rough Auckland neighbourhood.
They shortly after fostered his younger sister and brother before also adopting three more children.
As European immigrants caring for Maori kids, they were highly regarded in the community.
Yet behind closed doors they drank a lot and, while warm on occasion, were also highly manipulative. They also welcomed the welfare cheques their foster children brought in, John says.
He says he and his siblings experienced rejection as state wards their whole life and desperately needed nurturing, but instead came to compete for their foster parents' attention.
First violation
It was the school holidays in 1977 when his foster mum first violated him on her bed.
She had spoken about a desire to be with a Maori before.
Within weeks they were having regular sex behind her husband's back. She picked him up from school at lunchtime or got him out of bed at night.
After being told by his foster parents that they hated him and rebuked as a troublemaker for his poor results at school, he now felt loved and the focus of his foster mum's attention.
"It was an incredible transition from being hated and then all of a sudden a year later finding myself in her bed," he said.
Yet he also felt guilt because his foster mum made him feel like their sex was his fault.
The "shame" and years of conditioning to think of welfare workers as the enemy, made it impossible for him to tell anyone about what was happening.
His foster mum could be ruthless too. When her father visited from overseas one Christmas and grew suspicious, she threatened to cut him out of her life if he ever questioned her about it again.
When John's sister later caught them having sex in the lounge, his foster mum booted her out of the home and demonised her as a problem child to welfare workers, he says.
His sister is now a "psychological mess".
And while his brother was not sexually abused, he also suffered and later became involved with gangs and landed in jail.
Ongoing impact
John's sexual abuse ended when he was 17, but its impact lasted much longer with his foster mum later not approving of his wife and continuing to deny what she had done to him as a child.
After one emotional breakdown, he tried to reconcile with her.
"I said I'd forgiven her and she basically said, 'What for?'" he said.
Urged on by a counsellor and his sister, John then went to police.
That the justice system failed him was a particular tragedy, Clearwater says, because men already had a tough time coming forward to report their abuse.
A similar story
Among all those seeking help from Clearwater's support group, Dancing with the Stars performer Aaron Gilmore is the only one to ever lay a complaint with police.
Similar to John, Gilmore had been abused by a woman he considered a second mother, while aged between 11 and 18.
Like John, he was also told by police they could not see what crime had taken place.
Yet, unlike John, Gilmore's partner became furious with police officers and stirred them into action.
Gilmore's abuse also happened in the 1990s when it was a crime for a woman to abuse a boy and after a high profile trial in 2003, his abuser, Hendrika Margaret Shaskey, was sentenced to five years' jail for sexual violation.
He is grateful he was able to deal with the abuse while young and move on, but fears others who are not able to move on or who are failed by the justice system will harm themselves.
"Frankly it's dangerous. It's not gonna surprise me if someone kills themselves from getting that kind of response," he said.
He hopes John can stick to his path.
"Whether that be a legal pathway or just knowing in himself that he is going forwards."
For his part, John is still seeking legal justice but does not know where to turn next to get it.