The father in charge of the party described seeing two gatecrashers “hurling” building material from the roof of a garden shed shortly before Liam was speared in the head with a construction rod.

“He had a rebar protruding from his head, in excess of 2.5 metres,” Philip Staples told the Downing Centre District Court.

“It was sticking perpendicular out of his head,” he said.

Liam, then 17, now suffers from permanent brain injuries as a result of the attack.

Mr Staples looked visibly shaken as he described how his son's 18th birthday party at Forestville on Sydney's northern beaches turned “chaotic” in January last year.

“Everything went beserk,” Mr Staples said.

“Building materials started to be hurled. I was actually struck in the back by several pieces.”

Not long after, the gatecrashers jumped the back fence of the house and into a reserve before running off.

That is when Mr Staples realised his son's 17-year-old friend had been speared in the head.

Mr Staples gave evidence during the trial of Jimmy Tupuo, 19, and another teenager charged with their involvement in Liam's injuries, which left him in a coma and fighting for his life.

One teenager, who cannot be named, has pleaded guilty to causing grievous bodily harm with intent.

He pleaded guilty to five other charges including reckless wounding causing actual bodily harm, two counts of assault, unlawful violence and one count of assault occasioning actual bodily harm.

Mr Tupou, who is on bail, has pleaded not guilty to recklessly causing grievous bodily harm in company, reckless wounding causing actual bodily harm and using unlawful violence.

The trial continues.