Former Otago University tutor Clayton Weatherston told police he had killed his ex-girlfriend and student because of the emotional pain she had caused him, the High Court has heard.

Weatherston, 33, killed Sophie Elliott, 22, in the bedroom of her Dunedin family home on January 9 last year as Miss Elliott's mother tried to force her way into the bedroom, prosecutor Marie Grills told the court today on the first day of Weatherston's murder trial.

Weatherston denies murdering Miss Elliott, but has admitted manslaughter. He says he was provoked by Miss Elliott, who attacked him with a pair of scissors, and lost control. But the prosecution says there were no words or acts that caused provocation or any loss of self-control.

Miss Elliott died after suffering 216 stabbing or cutting wounds.

Mrs Grills said Miss Elliott suffered clusters of stab wounds to both eyes, her genitals, her breasts, her left cheek, her left temple and her left ear and the left side of her neck as well as 45 wounds to the front of her throat.

Her ears and the tip of her nose were cut off and several pieces of her hair were also cut. Mrs Grills said the nature of the attack indicated areas of Miss Elliott's beauty or attractiveness were being targeted.

A pathologist who examined her body would give evidence that these injuries indicated a "persistent, focused and determined attack".

In her opening address today to the court, Mrs Grills said the court would also hear of a previous assault by Weatherston on Miss Elliott in December, 2007, and Weatherston making comments to Miss Elliott that he wished she was dead and hoped a plane she was to travel on would crash.

A relationship began between Miss Elliott, an Otago University economics student, and Weatherston in June, 2007.

This relationship was regularly "up and down" and "all on or all off". By mid-December 2007, the relationship was all but over, Mrs Grills said.

On January 9, 2008, Miss Elliott was in her bedroom packing for a move to Wellington where she was due to start a new job with the Treasury.

Weatherston drove to the house that day with a knife in his computer bag, Mrs Grills said.

Miss Elliott's mother, Lesley Elliott, answered the door to Weatherston and he told her he wanted to see Miss Elliott and had something for her.

Weatherston went up to Miss Elliott's bedroom, and Mrs Elliott listened for loud noises from her daughter's room. She heard words to the effect of "don't Clayton" or "stop Clayton", Mrs Grills said.

Mrs Elliott initially tried to open the door to the bedroom but couldn't. When she did get the door to the bedroom open, she saw her daughter lying dead on the floor and covered in blood.

Weatherston was straddling Miss Elliott's body and stabbing her, Mrs Grills said. Weatherston then pushed the door closed.

Mrs Elliott telephoned the police. Officers arrived and had to get Weatherston to open the door, Mrs Grills said.

When the first policeman on the scene asked Weatherston what had occurred, he told the policeman: "I killed her" in a calm, normal tone, Mrs Grills said.

Asked why, Weatherston stated: "The emotional pain that she has caused me over the past year".

Weatherston's lawyer, Judith Ablett-Kerr, in her opening statement to the court, said the case was not a "whodunnit", as Weatherston did commit the act against Miss Elliott.

The issue was whether Weatherston was a cold-blooded, premeditated killer or a man who, as the result of provocation, lost the power of self-control.

The defence case was that he was provoked by the emotional pain of a torrid and tumultuous relationship with Miss Elliott, which he was ill-equipped to deal with. Miss Elliott had also attacked Weatherston with a pair of scissors on the day of the killing, Mrs Ablett-Kerr said.

The first of 31 prosecution witnesses has been called today to give evidence in the trial.

The trial, before a jury of eight men and four women, is expected to last about three weeks.