"He was paid by the state to care for these boys, and instead he exploited them for his own sexual gratification."
Opetaia has pleaded not guilty to all charges, and denies any of the events occurred.
His defence counsel Anoushka Bloem said Opetaia - who was also a husband, father to two children, gym instructor and boxing trainer - denied the charges because they were not true.
"The defence says these are fabrications, not true, they are claims for money. Opetaia stands innocent, a person falsely accused by these eight men."
Opetaia worked as an approved caregiver between 2001 and 2006, during which time he ran a residential facility with boxing gym attached where about 150 boys came into his care.
He lived at the facility, with the boys staying in a separate unit. A boxing gym was beneath the facility, where Opetaia would train the boys as part of his care work.
Archibald told the jury during the trial each man would tell a story of how Opetaia, a trained boxer, threatened to assault them, pitted them against each and other supplied alcohol and cannabis to them, and on one occasion methamphetamine.
He took advantage of them - many under 16 years of age, she said, with allegations stemming from inappropriate massages, to sexual violation and rape, with drugs often used as reward.
"Each charge represents a moment in that boy's life he would rather forget," Archibald said.
"Each complainant is a male, young and in a vulnerable position when they came into his care.
"Many had issues at home, some running away and into trouble.
"Each say Opetaia sexually abused them. Some were abused in the shower, some while in bed, some at night, some in his private unit."
Archibald said Opetaia was able to keep his victims quiet due to running a home full of "violence, fear and abuse".
After allegedly threatening to kill one boy and indecently assaulting him in the shower, Opetaia said: "I get what I want, when I want".
Archibald also disclosed to the jury Opetaia pleaded guilty to and had been convicted of a sexual offence of a teenage boy in 2014.
The 2014 conviction led to a mass investigation, where police attempted to speak to the 150 boys who had been in his care.
Archibald also mentioned allegations made against Opetaia of similar offending in the 1980s, which she said amounts to a pattern of offending.
In her opening remarks Bloem said Opetaia had become a caregiver because he wanted to help these young men, with upbringings similar to his own.
"He himself was from a difficult upbringing, where abuse was common. He wanted to help these young men, offer them a different life, offer them a better life."
Through the boxing gym were "many success stories", of young men turning their lives around - stories that were "incompatible with these complaints, because they are not true".
"He did not abuse them, he helped them," Bloem said.
The defence case was that "money and greed" were behind the allegations, Bloem said.
The 2018 police investigation only started after the 2014 conviction, she said.
Opetaia was a "low-hanging fruit, ripe for false allegations", she said.
"Not one of these made a complaint until police came knocking at the door."
She alleged many of the complainants - some of whom were in prison - were seeking financial compensation, and could even be working together.
"This is a well-known scheme in the prison environment."
The trial, before a jury and Justice Ian Gault, is set down for six weeks.
Opetaia is best known for caring for Liam Ashley before the 17-year-old was assaulted by George Baker while both were being transported to Auckland Central Remand Prison in 2006. Ashley died in hospital the next day.
Where to get help:
• If it's an emergency and you feel that you or someone else is at risk, call 111.
• If you've ever experienced sexual assault or abuse and need to talk to someone call the confidential crisis helpline Safe to Talk on: 0800 044 334 or text 4334.
• Alternatively contact your local police station.
• If you have been abused, remember it's not your fault.
