A 17-year-old boy has been found guilty of spearing teenager Liam Knight in the head at a party on Sydney's north shore.

Mr Knight was at an 18th birthday party at Forestville last year when a group of gatecrashers turned up and began causing trouble.

The teenager, who cannot be named, threw building materials including bricks and a steel pole from the roof of a shed.

The pole, which was almost three metres long, pierced Mr Knight's skull, causing him permanent brain damage and left him in a coma for two weeks.

Mr Knight entered Sydney's Downing Centre District Court with a walking stick and his family by his side to hear the verdicts for the two males accused of injuring him.

One of them, 19-year-old Jimmy Tupou, was found not guilty of the attack.

Judge Ronald Solomon found although he was on the roof at the time, witness accounts of what happened were inaccurate.

But he found the 17-year-old was responsible, saying he was in an aggressive state of mind at the time, and was heard swearing from the roof.

Last year, Mr Knight told Channel Nine's 60 Minutes program he could not remember what happened that night.

"I'm thinking my brain blocked that out because if I remembered that...oh I don't know how I would cope with that," he said.

He also now suffers from epileptic fits.

The 17-year-old boy will be sentenced in June.