Seven teenagers are in hospital with stab wounds after brawling outside a Chatswood house in the early hours of Saturday morning.

Police were called by concerned members of the public to a house on Mowbray Road just after midnight after a brawl broke out between two groups of people.

Detectives are investigating how it began, including whether the two groups were known to each other, but several witnesses are refusing to co-operate.

Five boys, aged between 16 and 19, suffered stab wounds to "numerous" body parts and were taken to Royal North Shore Hospital, while two 17-year-old boys were taken to St Vincent's Hospital. Their injuries are not life threatening.

Once the victims arrived at Royal North Shore they began fighting again, said Inspector Dave Vandergriend, Duty Officer for the North Shore Local Area Command.