Michael Basanovic has pleaded not guilty to Zeljko Mitrovic's murder.
Security footage appears to show a man turning to his son,
looking at his face and giving a nod.
Crown prosecutor Siobhan Herbert told a NSW Supreme Court jury that seconds after Michael Basanovic allegedly nodded at his son Wade, the younger man started firing a gun.
Zeljko "Steve" Mitrovic, a senior member of the Hells Angels outlaw motorcycle gang, died when three shots hit him in the chest, after a heated conversation with the Basanovics and their friend Brian Mcgarvie Brown.
Michael Basanovic, 49, Wade Basanovic, 24, and Mr Brown, 35, have pleaded not guilty to the murder of Mr Mitrovic.
An artist's impression of slain bikie Zeljko Mitrovic froom 2007.
The court heard there will be evidence the men had argued with Mr Mitrovic in the office of his transport business in Wetherill Park on the afternoon of January 15, 2013.
The argument followed a confrontation between Mr Mitrovic and Frank Bacic, an associate who claimed Mr Mitrovic owed him money, Ms Herbert said.
The jury was told on the first day of the men's trial on Tuesday that Michael Basanovic and Mr Brown were part of the Hells Angels outlaw motorcycle gang until 2011. There was no evidence Wade Basanovic was involved with the gang.
Defence barrister Troy Edwards, for Wade Basanovic, said there would be no dispute his client shot Mr Mitrovic.
But, Mr Edwards said, Mr Basanovic did so with the belief Mr Mitrovic had beaten his father and organised to take a "hit" out on him.
"He believed Mr Mitrovic was a person who could make that happen," Mr Edwards told the jury.
"He believed the police would be unable to protect his father.
"He believed the only way from stopping it ... was to confront Mr Mitrovic to call off the hit."
He said Wade Basanovic carried a gun because he feared he would be killed.
Mr Mitrovic reiterated his threats against Michael Basanovic that afternoon, Mr Edwards said.
"[Mr Mitrovic] said something like 'You're all f---ing dead' and he killed Mr Mitrovic and he believed it was the only option he had."
The confrontations and the shooting were captured on security cameras in Mr Mitrovic's office at Dynamic Transport, and there was no dispute the three men were there, Ms Herbert said.
The Crown alleges the men were part of a joint criminal enterprise.
Barrister Winston Terracini, SC, for Mr Brown, told the court his client was not accused of shooting or harming anyone.
"Mere presence while a criminal offence is taking place is not enough to make you a part of a joint criminal enterprise," Mr Terracini said.
He told the jury that while they might have opinions about outlaw motorcycle gangs, they needed to focus on the evidence before the court.
The trial continues before Justice David Davies.