Harris said he was drinking moonshine with Retimana, a woman and another friend, Wilson Hipango, at a campsite on Pine Beach off River Rd on December 24.
At some point in the night the woman has put herself to bed because she was drunk and Retimana had gone over to her.
Harris said he started to get upset when Retimana mounted the woman while she lay still.
Harris then went over and pulled Retimana off the woman and "hooked him".
"I let loose with a good punch to the nose... and he thanked me for punching him to snap him out of him."
He said told Retimana there was no need for that behaviour while they were there.
They continued drinking for five or 10 minutes and then Retimana went back over to the woman and mounted her again before offering a threesome to Harris.
Harris told the police he was disgusted because "it was the wrong time and place for that" and because he heard no consent given by the woman.
Harris said he saw red and gave Retimana "a couple of cracks".
"Because he did it twice I gave him a bigger crack. I think I used my knee and gave him another uppercut."
It was at this point Retimana tried to escape "probably so he wouldn't get hit again," Harris said.
Harris told the detectives he tried to pull Retimana away from the water before he fell in but instead Retimana jumped in the Waikato River and swam away.
Harris said he walked down the river edge and didn't see where Retimana came out of the water, but did see him climbing up some logs near the Boundary Road bridge.
When he saw Retimana was balancing on the logs and was safe, Harris walked away and said that was the last he saw of him.
The trial continues.
