The day before Emily Longley's death, her boyfriend Elliot Turner shocked friends by pretending he had "murdered her".
Turner told two teenagers he had smashed Emily over the head seven times with a hammer until he "heard her skull crunch".
Jasmin Snook, 19, told a jury at Turner's murder trial yesterday she burst into tears when she was taken in by Turner's lie in the early hours of May 6 after a night at club Klute.
The British court was also told that days before her death, Auckland teenager Emily slept with Turner's best friend.
Emily, 17, had sex with Luke Ashford shortly before she was allegedly strangled by Turner, 20, the son of a wealthy jeweller, in a jealous rage, a jury at Winchester Crown Court heard.
Turner is accused of strangling Emily, an aspiring model, at his parents' home.
His parents have been charged with trying to help him cover up the murder.
Emily was found dead in Turner's bed at the house in Bournemouth, Dorset, on May 7 last year.
Snook said: "In the early hours of May 6, after a night out in Klute, Turner texted me to say he had done something stupid.
"He turned up at my house about 3.05am and I went outside and talked to him on the pavement. He was near tears. He said 'she is dead'. He then started crying hysterically."
Snook said that one of Turner's friends, Oliver George, then turned up.
She added: "Turner told Oliver 'she is f******g dead, I killed her, I smashed her seven times over the head with a mallet. I heard her skull crunch'.
"He said it happened outside Klute in the carpark and he threw her body in the bushes outside.
"He then burst out laughing and said 'I'm f******g joking'."
Snook said Turner's friend Tom Crowe turned up and Turner said: "Emily is safe at home sleeping - but in a parallel universe I would've killed her."
Snook added: "I then burst out crying. I got back in my house at about 4.30am. I had a Facebook notification from Tom which was titled 'Dead'.
"It said 'Emily's body has just been taken past Klute in an ambulance'. What the f**k did Elliot do?'
"I replied, 'that is not funny, I feel really sick'."
The trial continues tomorrow.
- Solent News Agency