The woman, whose details are suppressed, told the court she had been drinking with Retimana for two days straight and was "wasted" that night because they were drinking moonshine.
The woman says she was tired and put herself to bed in her campsite near the Waikato River and then Retimana came over to her.
She said he pinned her arms above her head so she couldn't move and tried to have sex with her even though she told him to stop.
The next thing she knew "a fist came flying to my face" and she was knocked out by Retimana.
She said he still performed sex acts on her when she was unconscious. "Even though I was knocked out I could still feel things. I could still feel his fingers," she said.
She wasn't sure if they had sex against her will but her pants were taken off her, something she hadn't done herself.
When she came to, she told Boot, she woke up sore and in the water and was rescued by a friend.
She saw Harris and she said he told her that he "didn't like how he [Retimana] was treating me so he threw him in the river".
When questioned by Boot the woman conceded she may have heard those words wrong.
Earlier the woman told the court Retimana forced himself on to her despite her saying no.
She said she was "wasted" but could still say no and then a "fist came flying" at her face.
When she came to Harris told her he didn't like the way she was treated and so he threw him in the river where he subsequently drowned - but the witness acknowledged she may have heard the words wrong.
The woman, whose name is suppressed, was the first witness to be called to the stand by the Crown.
She described Retimana as a good guy but was "a bit rough" when he drank.
On the night of December 24 the woman told Crown lawyer Ross Douch they had a campsite on Pine Beach at the river side and she was drinking moonshine with Retimana and Harris.
She says they had been drinking for two days straight and were "tired and wasted".
When asked if any sexual advances were made by Retimana she said she told him to stop and then blacked out after being punched in the face by him.
When she came to she says Harris told her he "didn't like the way he [Tama Retimana] had been treating me so he threw him in the river".
The Crown alleges the Harris "lost it" after being invited to join in group sex acts.
Douch opened for the Crown this morning.
Douch said Harris and Retimana had been drinking moonshine on December 24, along with a woman.
Harris became angry when Retimana and the woman started being sexual in front of him, causing Harris to punch the victim and tell him to stop.
They went back to drinking moonshine but later Retimana again engaged in sexual activities and this time asked Harris to join in.
Douch said it was at this point that Harris "lost it" and he punched and kneed Retimana in the head.
Retimana then ran and tried to escape by jumping into the Waikato River.
He wasn't seen until three days later when a passing kayaker found the body on December 27.
A toxicology report found Retimana had 360 milligrams of alcohol in his blood, seven times over the legal limit to drive.
Douch told the jury the big question they would have to ask is whether Retimana jumped into the water through fear or threat of violence, something the Crown alleges.
The trial is set down for five days.
