River murderer claims not the only killer

A man has pleaded guilty to murdering a Perth woman whose body was found in the Canning River, but he claims he wasn't the only killer.

Peter Michael Bradshaw was due to face trial over the April 2013 death of Janelle Kathleen Knox but he instead pleaded guilty on Monday.

Bradshaw's defence lawyer told the Western Australia Supreme Court a second man placed a rope over Ms Knox's neck from behind, then passed it to Bradshaw as Ms Knox fell forward and began to choke.

The second man then stepped on Ms Knox's feet, grabbed one end of the rope and pulled it even higher.

The prosecution rejected those versions of events, stating Bradshaw had told police that the second man had nothing to do with the murder.

A trial of issues is being held so the judge can decide what actually happened before sentencing Bradshaw.