Police and ambulance paramedics responded to a Triple 0 call at 11.16am on Saturday, with reports a man, 41, had fallen from the third floor window of a Todd St building after an alleged assault inside an apartment.

Lake Illawarra police inspector Steve Johnson said the injured man was involved in an altercation with another man inside the unit before he climbed onto a window ledge.

He then slipped and fell 12-15m onto a concrete ramp below.

Inspector Johnson described the man’s injuries as ‘‘fairly serious’’, saying they included spinal, pelvic and limb injuries and ‘‘a fractured skull’’.

He remained conscious throughout the ordeal, paramedics said.

The man was taken by road ambulance to a waiting rescue helicopter, where he was flown to St George Hospital in a serious but stable condition.

‘‘Any witnesses to the incident are urged to contact Crime Stoppers,’’ Insp Johnson said.

He would not elaborate on the extent or reason for the alleged assault inside the apartment, nor would he say if any weapon was involved.

A crime scene was established at the unit block.

Insp Johnson said detectives would this afternoon interview the injured man in an attempt to find out what happened.

Initial reports that the man had been stabbed before falling remain unconfirmed.