SYDNEY, Australia - A young Chinese woman fell naked to her death from a third-floor balcony while trying to escape a knife-wielding intruder who had forced himself inside her apartment, Australian police said Monday.

The body of the 18-year-old woman was found on the ground outside her inner-city Sydney residence on Sunday afternoon.

Her 19-year-old Korean boyfriend, who was also naked when he tumbled from the balcony of the apartment onto a concrete landing, was in hospital Monday with fractured vertebrae, a broken pelvis and two broken legs.

"It's believed the deceased and the injured male were trying to get to the next level ? from level three to level two ? to try and get away from the offender," New South Wales police superintendent Luke Freudenstein said.

Freudenstein said it appeared that the intruder followed a woman who was visiting the couple into the security entrance when she was buzzed in and then held a knife to her throat and demanded she let him into the apartment.

He was allegedly inside the home for an hour before the couple tried to escape and now remains on the run.

Freudenstein said police were still trying to piece together the events which led to the unnamed woman's death but that they believed the armed man did not know his victims and was acting opportunistically.

"Something's happened in that unit to make them take that step, 25 metres (82.5 feet) above the footpath, for them to try and climb to the balcony beneath," he told reporters. "It is a tragic incident."

The dead woman and her boyfriend had been studying in Australia. Their flatmate, a 20-year-old Chinese woman, was uninjured in the incident.