The words are burned into witness Leonie Cann's brain: "You f***ing c***. If I wasn't wearing this bracelet, I'd f***ing kill you."

Ms Cann told Campbelltown Local Court on Tuesday she heard Ahmed Elomar, 34, threaten a fellow spectator at a junior soccer match in Sydney's southwest in May 2017.

"I was really shocked ... he looked very aggressive," Ms Cann said.

What she didn't know at the time was that Elomar was the brother of slain Islamic State fighter Mohamed Elomar.

He was on parole - after assaulting police in 2012 during the Hyde Park riots - and wearing an electronic monitoring bracelet when he attended the May 13 match between St Marys and Narellan in Raby.

Ms Cann told the court there was a melee after the soccer game when some of the players refused to shake hands.

Shortly afterwards she saw Elomar yelling and threatening one of the Narellan fathers.

Under cross-examination from Elomar's lawyer, Kirk Dailly, Ms Cann said those words were burned into her memory.

"I believe what I heard was true. I am certain."

Mr Dailly argued there were inconsistencies between what Ms Cann said and what another witness had earlier told the court.

But Magistrate David Degnan found Ms Cann to be believable.

"I was quite impressed with her," he said.

Elomar will spend at least seven months behind bars for the threats after being found guilty by Mr Degnan. With time already served he'll be eligible for parole in June.

Elomar's lawyer told the court his client suffers from dementia, as a result of his boxing days, and post-traumatic stress disorder.

Mohamed Elomar is believed to have been killed fighting with Islamic State in the Middle East.