Crown prosecutor Steve Manning said they wore condoms because they were aware of the risk of leaving DNA evidence, but were not sufficiently "clued up" to know they could leave some from their saliva. DNA later found on the victim's bra matched that of Skipper.
Skipper claimed it came from the morning after the sexual assault - which he said he had nothing to do with - when Karetu and the victim came to his house and they kissed, and he had kissed her on the breasts. She said that had never occurred.
Mr Manning told the jury the DNA evidence was "on its own enough to completely reject Mr Skipper's evidence", and Skipper had invented the story because he was "stuck between a rock and a hard place".
"What happened at the river is clear. Mr Skipper is responsible for those three sexual offences and Mr Karetu, his younger friend, was a party to that," he said in his closing address.
Karetu was found guilty despite not taking part in the sexual assault because he had been a party to it. He facilitated it by picking up the offenders and sat in the car while it occurred.
After the assault Skipper and the two other offenders were dropped off, but the victim spent the rest of the weekend drinking and taking drugs in Karetu's company.
Karetu was also found guilty of assault on a female and threatening to kill.
They were both remanded in custody for sentencing on October 12.
