The alleged victim of a violent assault over an ex-girlfriend suffered ''life threatening'' injuries, the prosecution says.
The accused man, Dean Law, has pleaded not guilty in ACT Supreme Court to recklessly inflicting grievous bodily harm.
Law and the alleged victim, Justin Roffey, were both former lovers of Belinda Chavasse.
The court heard Law went to a home in Wanniassa on December 14 last year to speak with Ms Chavasse, who had a month earlier ended their romance.
Ms Chavasse told Law to leave and went to re-enter the home via the front door and backyard but he blocked and grabbed her.
The court heard Roffey came to the aid of Ms Chavasse armed with a metal pole as a weapon.
The two men fought and Law ended up pinning Roffey to the ground and repeatedly punching him in the head.
Ms Chavasse attempted to intervene before Law stood up and left in his car.
The accused man's barrister told the jury that Law had acted in self-defence.
But prosecutor Anthony Williamson, in his opening statement, said the measure or level of force was not necessary.
Mr Williamson said the alleged victim suffered fractures to his skull, face and ribs.
The Crown said Mr Roffey still endured pain, scarring and eyesight problems.
He said doctors treating the victim considered the injuries life threatening and needing surgery.
The prosecution is expected to call medical evidence on Wednesday to attest to the severity of the alleged victim's injuries.
But Law's barrister, James Lawton, said the accused had acted in self-defence.
The defence said Mr Roffey had struck the first blow, hitting Law, who ducked, on the back of the head with the bar.
Mr Lawton told the court the accused continually told the alleged victim to ''drop the bar'' during the incident.
''[Law] is not the one that used a metal pole,'' Mr Lawton said.
''[Think] what you would do if you were assaulted with a metal pole.''
Mr Lawton told the jury the two Crown witnesses had lied - either by omission or deliberately - to police when making their statements.
The accused is expected to enter the witness box when the trial, before Justice John Burns, continues on Wednesday.