By Samuel Roberts
A 21-year-old man has been arrested after three teenagers were stabbed at a birthday party on Sydney's upper north shore on Saturday night.
One of the victims, a 16-year-old boy, is in a critical condition while a 17-year-old and 18-year-old are in a stable condition. All are at the Royal North Shore Hospital.
Police were called to Heydon Avenue, Warrawee, just after midnight after an intruder gatecrashed the house party with a knife.
"He just went crazy," the owner of the home said.
The Heydon Avenue house where the stabbings took place.
The man said the party of approximately 60 guests had been under control until the intrusion.
A resident said a large group had been loitering outside the house throughout the night.
"They weren't causing a disturbance, but you always feel on edge with a group that size of young males," he said.
There were about 50 people in the group, a witness said.
"It appeared like they hadn't been let into the party, maybe they just heard about the party through social media," the witness said.
The group stayed in the street outside the party but were not reported to be drinking.
"There was no great amount of alcohol. When I checked the street in the morning there were mainly soft drink cans around," a neighbour said.
The neighbour said he was shocked by the attack.
"It's completely out of the blue. Before this, crime in Warrawee was a train running late," he said.
Other residents reported hearing very loud noises from 7pm onwards and seeing shirtless men on the balcony of the house.
Two security guards had been on duty for the party, but had been unable to prevent the armed man from entering the house.
The 21-year-old man was arrested near Hornsby police station on Sunday morning.
He has been charged with three counts of wounding a person with intent to cause grievous bodily harm and one count of possessing a weapon without a permit.
He was refused bail and will appear at Hornsby Local Court on Monday.
WITH AAP