Emergency crews were called to Perth's Curtin University shortly before 13:00 local time (05:00 GMT) on Tuesday.
Aerial footage taken by local media showed a smashed glass roof and a large pile of rubble on the ground.
A 23-year-old man died immediately after falling more than 20 metres, authorities said.
St John's Ambulance said another two men in their 20s had "multiple injuries" and were being treated at Royal Perth Hospital. Nobody else was trapped.
Curtin University said the collapse happened at a building that was under construction by property group Lendlease.
"We have been advised that no Curtin students or staff were involved in the incident," the university said in a statement.
"Our thoughts are with all impacted by this accident."
The cause of the accident would be investigated by a work safety regulator, authorities said.
Electrical Trades Union official Damian Clancey told the Australian Broadcasting Corporation he had heard a "massive crashing sound".
"It sounded like smashing glass - someone filling a skip bin and you were standing three feet away from it. It was a tremendous noise."
The university urged people to avoid the area.