The former boyfriend of Aucklander Emily Longley joked with friends that he'd killed her with a hammer two days before her death.
The 17-year-old was found dead in Elliot Turner's bed at his parents' house in the south of England.
They're accused of helping him cover up her murder.
The New Zealand Herald reports that Turner's friend Tom Crowe has told a trial at the Winchester Crown Court the pair drove to a nightclub where Emily Longley was on a date with another boy.
He says Turner emerged saying he'd hit her six or seven times with a hammer and killed her.
Turner later said he was only joking. He stopped crying, was cheerful and was jumping up and down.