As his partner lay dead and bloodied in a bed nearby, James William Te Hiko called police.

"I have killed her with my hands and feet. I have beaten and killed her," prosecutors say he told the person on the line.

The 44-year-old Atiamuri scaffolder is on trial in the High Court at Rotorua accused of murdering partner Queenie Karaka, generally known as Nina Thompson, in a sustained beating during a 10-hour period on April 19 and 20 last year.

In her opening address today, Rotorua Crown solicitor Amanda Gordon said the couple had been the only people in the defendant's home at the time Ms Thompson died, meaning it would never be exactly known what happened.

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But a large metal pipe had been found in the house with Ms Thompson's blood and hair on it.

"We also know that while the focus of the defendant's attack was on her head and face, she suffered extreme trauma to just about every part of her body, that she was rendered unconscious," Ms Gordon said.

She described the injuries as so severe Te Hiko's brother, who'd been with the couple earlier on April 19, had barely been able to recognise her swollen face.

There were loud sobs from the public gallery as Ms Gordon outlined the injuries Te Hiko inflicted.

Ms Gordon said hand and arm injuries indicated Ms Thompson had tried to defend and protect herself. She said her blood-soaked and torn T-shirt had been found on the bedroom floor and when she was found she had clean clothes on.

"By the end of this case you will know from the evidence that he left her to die in the bed," she said. Blood stained towels indicated Te Hiko had attempted to clean up some of the mess in the room.

The jury heard how Te Hiko had phoned his mother at 7.15am on April 20 mumbling "mum, mum, mum, I did it, I woke up and she wasn't breathing". His next call was to the police.

When his brother arrived at the house Te Hiko hugged him, telling him: "I couldn't stop, I am sorry, she is dead, I killed her."

Te Hiko's lawyer, Harry Edward, told the jury his client admitted he had killed Ms Thompson but that he was guilty of manslaughter not murder.

"He is saying something horrible happened but he didn't intend to kill her with the injuries he inflicted," Mr Edward said.

Describing the phone call she received from her son, Lexi Te Hiko said she had never heard his voice like that before.

He was "gone", crying and unable to speak most of the time, she said.

She told Mr Edward once an ambulance took her son's partner from his home, she and another son joined Ms Thompson's family for a service in the driveway.

She agreed she told police the couple's relationship was topsy-turvy.

Tokoroa family violence officer Andrew Munro said Te Hiko had been matter of fact, rather than distressed, when he called him on his cell phone to say he had bashed and killed Ms Thompson after they had been drinking and arguing.

Later in the conversation, he started yelling and became hysterical, saying he had woken up and found her dead in their bed.

When he saw Te Hiko at his home on Atiamuri's Ongaroto marae, Sergeant Daniel Peat said he told him Ms Thompson wouldn't wake up and that he'd bashed her the previous night.

When the sergeant hadn't been able to find a pulse, he gave her CPR but the only response was gurgling.

Her body was covered with extensive bruising and both eye sockets were blackened.

The trial continues.