A love rival has told the Claremont serial killings trial he had sex with Bradley Robert Edwards' first wife as the accused slept in another bedroom.
Edwards, 50, is accused of murdering Sarah Spiers, 18, Jane Rimmer, 23, and Ciara Glennon, 27, in the mid-1990s during his marriage breakdown after his first wife had an affair, which resulted in the birth of a child.
The ex-Telstra technician's love rival, whose identity is suppressed, testified in the Western Australia Supreme Court on Monday via video link from overseas, saying he did not initially know the woman was married.
He said she invited him and his children to go horse riding, then he asked her out to dinner and they saw each other when they could.
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"I believe we were both in a situation where we were craving something," he said.
"I didn't really have anyone that I could connect with.
"I wasn't intending to break anyone's marriage."
The man said the wife came to his home about half a dozen times and on one occasion she had car trouble, requiring Edwards to help jump start the vehicle.
After the man moved in as a boarder, the affair continued.
"She would sneak into my room while Bradley was still asleep," he said.
"I thought she was playing a dangerous game."
His evidence is in contrast to the first wife, whose identity is also suppressed.
She testified last week that the man was a close friend and they started a sexual relationship towards the end of 1995 after he moved into the marital home.
But the man did back up her evidence that Edwards spent hours on the computer alone.
Edwards and his first wife separated between late 1995 and early 1996.
She then moved in with the boarder.