A Melbourne woman accused of killing her partner had struck up a relationship with a "besotted" personal trainer who allegedly planned to kill his rival, a court has been told.

The woman, who can't be named for legal reasons, is on trial in the Supreme Court of Victoria for the May 3, 2005 murder of her partner, whose body has never been found.

It is alleged she complained to her friends and lovers about her partner, claiming he was controlling, violent, had raped aspokercc her and had locked her in their home.

It is also claimed she told friends she planned to kill her partner by shooting him under the cover of New Year fireworks with a purse pistol she kept in her bungalow.

On Friday, the woman's former employer gave evidence via video link from Las Vegas.

The witness said she introduced the woman to a "good-looking", leather-pants-clad personal trainer at their workplace.

The personal trainer "went mental" and was "into her, big time", the employer said.

The pair struck up a relationship while the woman was still living with her now-deceased partner at their Reservoir home, she added.

The employer subsequently quit her early morning personal training sessions when she "started getting frightened" being with the man.

"He was completely besotted with her. He was into her big time. He just wanted to spend time with her and see her," the employer said.

"A lot of the time he was really upset and angry - I mean sobbing ... basically didn't like (the woman's partner), didn't like that she was locked up and wasn't let out.

"He said he wanted to kill (the man)."

Crown prosecutor Andrew Tinney SC previously said the woman either killed her partner herself or took part in a "joint criminal enterprise" with one or more others.

He claimed she boasted about murdering the man with a fire poker as he slept and told another person she slit his throat while they had sex and "it was the best orgasm I ever had".

Mr Tinney also said the woman told another lover she had help disposing of the man's body, dismembering it and dropping it into Port Phillip Bay.

But defence barrister John Kelly SC said on Friday that "every one of those allegations is denied", adding that the woman had nothing to do with the man's disappearance.

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