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.
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.