The crimes included 18 counts of indecency, two of rape and one of indecent assault.
The defendant's sexual offending spanned the period of 1963 to 1983 but only came to light when one of the victims came forward in early 2015.
Other women were interviewed by police over the course of the year and as a result the man was charged at the start of 2016.
One witness told the jury at trial she had been violated while the defendant's wife was in hospital giving birth.
Others spoke about Kannewischer repeatedly groping them.
One victim said the defendant would enter her room late at night and fondle her "quite roughly" on a regular basis.
"It seemed to go on forever," the woman said.
Another said after she was molested, Kannewischer forced her to lie on the floor where she was then raped.
Multiple victims said the defendant told them the lewd behaviour would help them "become a woman" and he would also warn them not to tell anyone about what had happened.
The victims read their emotional impact statements in court this morning.
"You stripped me of my childhood," one woman said.
She described a multitude of psychological problems suffered since the prolonged ordeal and said she had attempted suicide in the past.
Other victims sought refuge in alcohol and overeating to numb their pain, the court heard.
One woman faced the man today, as he sat in the dock, with one question: "Why?"
"The trial brought back many hidden nightmares, so the tears once again have flown freely and black thoughts have come back," another victim said.
But she was resolute and said she would get through the trauma with the help of the other victims.
Judge Crosbie praised them for speaking "bravely and candidly".
"You are a strong formidable group of women and you are all survivors." he said.
He addressed Kannewischer: "Deep down, I hope that what you heard in court today was hard to listen to."
Crown prosecutor Craig Power said there was "an aspect of brazenness to the offending", which often took place when there were others present.
"The effects have been lifelong and profound," he said.
Power noted Kannewischer had no remorse and was assessed by Probation as having minimal insight into his behaviour.
He had told Probation one of the victims was his "favourite" while another he labelled a "schemer", the court heard.
"There is a degree of flippancy and avoidance," the prosecutor said.
Defence counsel John Westgate said his client maintained his innocence, which explained the perceived lack of contrition.
While Kannewischer's physical health was poor, Judge Crosbie said his cognitive functioning was not impaired and the man knew the gravity of the situation.
If you're in danger NOW:
• Phone the police on 111 or ask neighbours of friends to ring for you.
• Run outside and head for where there are other people.
• Scream for help so that your neighbours can hear you.
• If you are being abused, remember it's not your fault.
Where to go for help or more information:
• NZ Police
• Help Auckland 24/7 helpline 09 623 1700
• Rape Prevention Education
• Wellington Help 24/7 crisisline 04 801 6655, push 0
• Male Survivors of Sexual Abuse
• Women's Refuge: Free national crisis line operates 24/7 - 0800 REFUGE or 0800 733 843 www.womensrefuge.org.nz
• Shine, free national helpline 9am- 11pm every day - 0508 744 633 www.2shine.org.nz
• Shakti: Providing specialist cultural services for African, Asian and Middle Eastern women and their children. Crisis line 24/7 0800 742 584
• White Ribbon: Aiming to eliminate men's violence towards women, focusing this year on sexual violence and the issue of consent. www.whiteribbon.org.nz
