Lesley Elliott today told the Christchurch High Court jury hearing Weatherston's trial for murder that she knew Sophie was dead when she saw her.
She was on her back in the corner, her face was deathly white, her eyes were staring and there was a lot of blood from one eye.
Mrs Elliott had run upstairs after hearing her daughter scream "Stop it Clayton, stop it", or "Don't Clayton, don't" but she could not open the bedroom door so ran downstairs to get a meat skewer to use on the lock.
Sophie's screaming then stopped and there was a rhythmic sound which she thought was a headboard banging against the wall.
"I thought Clayton was raping her," Mrs Elliott said.
But when she got the door open and stepped inside, she saw Weatherston (33) straddling Sophie and continuing to stab her.
"His arm was going up and down. I thought he was stabbing right through her. That was what the rhythmic noise was."
Mrs Elliott said she was screaming and Weatherston leaned over and closed the door in her face.
She thought she went to the study to make a 111 call but was now aware she was already on a cell phone making the call when she had got the bedroom door open.
Clayton Robert Weatherston (33), with whom Miss Elliott was in a relationship for several months in 2007, has admitted killing the graduate economics student on January 9 last year but claims it was manslaughter, not murder.
In earlier evidence today, Mrs Elliott told the jury Sophie had been upset and tearful when describing assaults on her by Weatherston, the most recent being two days before the fatal January 9 attack.
Her daughter had gone to the university to give Weatherston a cheque for repairs to a glass door pane she had broken 10 days earlier at his flat.
Weatherston saw her and came after her, saying " we need to talk".
They were in his office and Sophie told him she was scared of him because of the earlier assault on the day the door pane was broken.
Sophie told her that when Weatherston denied the earlier incident, she "lost it" and did to him what she said he had done to her - pushing her arm against his neck with a hand over his mouth, Mrs Elliott said.
And when she told him she had been advised to go to the police over the earlier assault, Weatherston's response was that he could not go to the police because of what Sophie had just done to him.
When Sophie was telling her mother about the incident, she was very upset and crying, asking "why does he hate me so much, I don't understand it", Mrs Elliott told the court.
And she said Sophie had also been quite upset when telling her about the incident at Weatherston's flat on December 27.
On that occasion, her daughter had gone to give him an album of graduation photographs she had created for him.
He had a present for her , a painting of the "Titanic", but she said there would not be enough room for it in her flat in Wellington.
As they were talking, Weatherston said "we've got to sort this out" but she told him "it's over".
He became amorous and suggested they go to the bedroom and have sex but Sophie said "you're obviously not getting the message, it's over".
Her daughter told her Weatherston then "turned nasty", Mrs Elliott said.
He grabbed her, threw her on the bed, putting his arm against her throat and his hand across her mouth.
Sophie told her mother she struggled and screamed and managed to get away.
She slammed the door on the way out, breaking one of the glass panes.
When Weatherston followed her out, she apologised and, he told her he had hoped the plane would crash when she was coming back from Australia and she would die.
Mrs Elliott said Sophie's friends had advised her to go to the police but her attitude was "what's the point", that she was leaving soon for Wellington.
Then she came home, burst into tears and told her mother what had happened, demonstrating what Weatherston had done.
"I also advised her to go to police," Mrs Elliott said.
