A Sydney teenager is being questioned over the gatecrashing attack of Liam Knight who remains in a coma after a metal rod was thrown at his head.

A 16-year-old boy from North Ryde handed himself into Surry Hills police station on Tuesday afternoon and is being questioned by police.

The victim, 17, remains in a critical condition in Royal North Shore Hospital after attending a friend's birthday party on Friday night at Forestville, in Sydney's north.

Around 50 people were enjoying themselves when a group of gatecrashers attempted to get into the home about 11.30pm (AEDT).

A fight broke out before one of the intruders went to the back of the house and climbed a fence where he found some building materials.

"He started throwing those materials at the crowd," Detective Inspector Craig Wonders told reporters on Monday.

"It was certainly the wrong place for Liam."

A metal rod penetrated his head so deeply part of it had to be cut off at the scene before he could be transported to Royal North Shore Hospital where doctors surgically removed it.

A police source told AAP that detectives identified the alleged offender as early as Saturday.

Liam's traumatised family was keeping a bedside vigil, police said.

His brother Shane Knight said he would miss his brother.

"I'm going to miss his humour and his smiles," he told Network Ten.

"I'm praying for him to get better."

A 16-year-old who also attended the party was walking home when a group of males punched him in the face.

He fled before raising the alarm and making a statement to police.