Claremont trial: Accused serial killer 'threatened to kill wife's lover over dangerous affair'

The mystery man who had an affair with accused Claremont serial killer Bradley Robert Edwards' first wife has told the court the Telstra technician threatened to kill him.

Giving evidence via video link from overseas, he spilled details of his secret relationship with Edwards' first wife - as their marriage unraveled. Both of their identities have been suppressed.

The man claimed he'd already been intimate with Edwards' first wife by the time he moved in with the couple around March 1995 and they continued having sex on Sunday mornings while Edwards was asleep.

He said Edwards' first wife said "Bradley won't wake up" and he thought she was "playing a dangerous game."

That was a direct contradiction from Edwards' first wife who previously told the court they didn't have sex until after she moved out.

The man said once Edwards walked in to see them kissing and he wanted to leave because he "feared for his life" but Edwards and his wife encouraged him to stay.

He said in a later telephone conversation Edwards threatened to kill him.

The court has previously heard Edwards and his first wife split in late 1995 or early 1996, not long before Sarah Spiers vanished from Claremont.

Prosecutors are trying to show Edwards attacked women he didn't know when he was experiencing turmoil in his private life.

Edwards has denied murdering Sarah Spiers, Jane Rimmer and Ciara Glennon in the 1990s.

Under cross examination, Edwards' first wife's lover was questioned about his memory of the timing of certain events, defence lawyer Paul Yovich pointing out he'd got the date of the birth of his child wrong.

The mystery man also agreed Edwards hadn't appeared angry when he walked in on them kissing.

The trial continues.