Weatherston (33), who denies murdering the 22-year-old honours student on January 9 last year, had spent two hours talking about Miss Elliott to a graduate student on January 8.
And there was a second conversation for about an hour over coffee late the next morning, which was Weatherston's 32nd birthday.
Within a short time of leaving the Commerce Building after that second conversation, Weatherston went to the Elliott family's home in Ravensbourne.
There he stabbed Sophie to death in her bedroom.
The Crown says the 216 separate wounds and the way they were clustered indicated a controlled and focused attack, targeting areas of Sophie Elliott's physical beauty and attractiveness.
But the defence says Weatherston was provoked into losing self control by the emotional pain of the relationship and by Miss Elliott attacking him with a pair of scissors.
In evidence on the sixth day of Weatherston's trial before Justice Judith Potter and a jury, graduate student and research fellow Sarah Forbes said Weatherston wanted to discuss Miss Elliott with a woman apart from his former girlfriend.
On January 8, he talked about her and the relationship for two hours, in what Miss Forbes said was a very negative conversation.
From what he said, she believed the relationship had ended.
Weatherston told her he wanted nothing more to do with Miss Elliott.
She advised him not to have any more contact and to remove his name from any websites.
Although she had met Miss Elliott only once, the young woman had subsequently invited her to be a friend on Facebook, Miss Forbes said.
When she told Weatherston that, he advised her not to do it as Miss Elliott would "monitor her" on Facebook.
"I knew amongst Weatherston's friends she was called the `psycho girlfriend'," Ms Forbes told the court.
She said Weatherston described to her how Miss Elliott attacked him on two occasions.
He said she had come to his office the previous day "shaking and sweating" and he had hugged her for 20 to 30 seconds.
As he was finishing the hug, believing she was calm, Miss Elliott had lunged across and struck him across the face with her elbow, saying "Now we're even".
Weatherston told her he did not react but left the room, Miss Forbes said.
He also recounted how, on an earlier occasion, Miss Elliott arrived at his flat and picked an argument, trying to hit him.
To stop her, he grabbed her and they both fell on the bed.
After he had stopped her hitting him, he got off the bed and told her to leave and she slammed the door so hard it smashed.
Weatherston told Ms Forbes he asked Miss Elliott to contribute to the repair of the door and when she brought a cheque to his office, she was screaming so loudly the whole department could hear it.
The accused said Miss Elliott told him she thought he was going to rape her.
Miss Forbes said Weatherston had asked her if he should be tested for a sexually transmitted disease as he knew Miss Elliott had slept with someone else while on holiday in Australia. From the question, she assumed Weatherston must have slept with Miss Elliott after she returned.
He also said he was not going to be "controlled" like Miss Elliott's previous boyfriend who, he said, was submissive and easy to control.
With reference to Miss Elliott's mental state, Weatherston told her Miss Elliott had wanted to commit suicide.
But she did not know when that was, Miss Forbes said.
Weatherston said Miss Elliott was insecure about the relationship, and she did "spot checks" at his place.
"He told me she'd go over to his house in the middle of the night and check there was no other woman there."
He told Miss Forbes he was in a new relationship and was very happy in it, and that he was looking forward to Miss Elliott going to work at the Treasury so that she would be out of his life.
During that conversation on January 8, Weatherston expressed his concerns about what Miss Elliott might be saying to her course supervisor, Robert Alexander, about him and whether that might affect his chances of getting a lecturing position with the department.
He said she had told him she was going to say as many nasty things about him as she could.
But when she saw him again the next morning, his birthday, Miss Forbes said Weatherston seemed positive, upbeat and happy.
He talked about being able to go back to work at the Treasury if he missed out on the lectureship.
And he made the comment they were "all weird" at the Treasury, so Miss Elliott would fit in well there.
Cross-examined by defence counsel Greg King, Miss Forbes agreed Weatherston told her he thought Miss Elliott had a borderline personality disorder.
And she agreed that, at Weatherston's graduation party, Miss Elliott had wanted to know why there were so many girls present.
She thought Miss Elliott was possessive and jealous and was bad-mouthing Weatherston at his own party.
