Ashley Arnopp, 20, was convicted of the December 23 murder by Justice Allan Mackenzie at the High Court in Palmerston North yesterday and remanded in custody for sentencing.
The change of plea came as the Crown's case was winding up and evidence from 28 of the 29 witnesses had been heard.
The plea change prompted angry outbursts from the public gallery that were quickly quelled by Justice Mackenzie.
In a statement given to Sergeant Steve Braybrook, Arnopp admitted to killing his "friend" during an angry, adrenalin-fuelled rage over money owed to him.
A fight began between the pair, deteriorating into an attack on Mr Waipouri over about an hour leading to his death.
A post mortem examination revealed Mr Waipouri died from head and neck injuries but had no defensive injuries.
The tip of Mr Waipouri's penis was cut off and he also had injuries to his ear and nipples.
The trial continues today for his co-accused Andre Gilling, 17, who maintains his innocence.
In a statement to Detective Marie Lamberth on December 28, Gilling said he'd never met Mr Waipouri until December 22, when the accused pair socialised at Mr Waipouri's Rangitikei Street flat. Gilling said he saw Arnopp kick Mr Waipouri, who had fallen asleep on the lounge floor.
After about five minutes Mr Waipouri no longer put his hand up to protect his head, Gilling said.
Arnopp had asked Gilling to punch Mr Waipouri. However, Gilling said he shook Mr Waipouri instead and urged him to get up.
Gilling's facial injuries arose when he tripped over Mr Waipouri's legs and smacked into a chair, giving himself a blackened eye and a cut nose.
He denied any further involvement in the attack and said he had "no reason to kill him" - Mr Waipouri had offered the pair food and accommodation.
Gilling said there were periods throughout the night of December 22 and 23, when he'd slept at the table.
Upon waking and seeing Mr Waipouri covered in blood, Gilling said he "freaked out".
"I knew it was wrong."
Gilling told police he was "too wasted" to do anything.
The blood found on his white socks was from an old abrasion.
In evidence, police medical officer Doctor Trevor Parry said the abrasion on Gilling's foot appeared to be a "fresh" wound.
Dr Parry said Gilling had a lot of blood on his hands and legs, and cuts to his hands, face and feet.
The trial of Gilling continues.