It was stuck so firmly, Mr Snape had to make the wound slightly wider in order to slide the knife out.
It would take a "considerable amount of force" for the knife blade to be stabbed in so far, he said.
"The blade of the knife went ... through the skull in the temporal region behind the eyebrow, and then it went into the back part of the eye socket ... within probably less than a millimetre or two of the optic nerve.
"If it had severed that, he would be blind."
The knife had to be removed along the same tract it went in so as not to create more damage, especially to the man's eye and base of the brain, Mr Snape said.
"If it had been posted half a centimetre further backwards, posteriorly, he could have sustained very serious damage to his brain stem or to his artery up to the brain.
"He could have sustained life-threatening injuries."
The victim is stable and is recovering at Christchurch Hospital today after delicate surgery to remove the weapon.
Mr Snape said the man was extremely lucky not to have been very seriously injured.
"He was unlucky to be where he was, but he was lucky because it could have been worse."
The incident happened during a party at the man's St Albans flat about 1am yesterday.
Witnesses had told Mr Snape the man was attempting to break up a fight when the incident happened.
Canterbury Police communications manager Stephen Hill said the police officer who was one of the first to arrive at the crime scene had emergency medical experience.
Constable Carl Christensen made a decision to leave the knife where it was until the man was at hospital. "If someone bumps the handle or something it could cause more issues inside the head, so I just had to use dressings from within our first aid kit to pack around the handle of the knife to stop it moving, and then just wrap a bandage around his head to hold all the padding in place so you couldn't wiggle the handle and cause any further damage," he told 3 News.
"That police officer is a hero; a super-hero," said Leon Fagan, the injured man's friend. He said he thought the victim was going to die. "His dad was sitting there holding him. You can imagine what was going through his head. Your son's in his arms, he's got a knife in his head. You can imagine what that would do to a man," he told 3 News.
The injured man would be staying in hospital for a few days, and staff would be checking no damage had been caused to his eye and there was no bleeding beneath the brain.
The man accused of the attack, also aged 20, appeared at the Christchurch District Court yesterday and was remanded in custody.
He is due to reappear tomorrow on charges of wounding with intent to cause grievous bodily harm.
