A VICTORIAN father began plotting the murder of Carly Ryan just days after the teenager spurned his romantic advances, the man's son has told a court.

The 22-year-old, who cannot be identified, today took the stand in the Supreme Court to give evidence against his father and younger brother, Adelaide Now reports.

He told the jury he rejected his father's requests that he join the vendetta against Carly, 15, and begged him via letter not to involve his other child.

Those pleas, he said today, fell on deaf ears. And his father and younger brother went to Adelaide, then returned boasting of their actions.

"My father showed me his knuckles and asked 'do these look bruised to you?'," he said.

"He told me that he had punched Carly Ryan in the face, that he had killed her, that he had 'done the job'.

"He said he had punched her in the face, pushed her face into the sand and, after that, he had thrown her into the water."

The father, 50, and younger son, 20, have each pleaded not guilty to murdering Carly in February 2007.

Prosecutors have alleged the father and his son used the "cyberspace alter-ego" of Brandon Kane to seduce the teen.

They claim that, after Carly spurned the father at her 15th birthday party, the duo used their "internet construct" to lure her to Port Elliot, where she was bashed and drowned.

Giving evidence today, the older son said his father was angry when he returned to Victoria following Carly's party.

"At first he said he wanted to go on a holiday, back to Adelaide, with me and my brother," he said.

"Later, he said he wanted to 'fix up' Carly and said other things that indicated he wanted to harm her.

"He wanted me to help him while he was there, to help him kill her.

"I didn't know what to do... I was stumped."

Shocked and upset, the older son moved out of the house but left behind a letter.

"I wrote that he wasn't the same person he was before, and that I couldn't believe he was trying to bring me and my brother into it," he said.

"I told him if that, if he kept going, he was going to lose his sons.

"The letter was pretty abusive... I was hoping I might persuade my father about wanting to kill her."

The son said his father and brother went to Adelaide and, upon their return, visited him at his new home and spoke about Carly.

"My brother said that he had hooked up with Carly, and at the time I took that to mean they had kissed," he said.

"My brother is quite boastful, and he would have mentioned if he had gotten any further than that."

The trial, before Justice Trish Kelly, is continuing.