SYDNEY, DEC. 8 -- A 22-year-old man armed with a rifle fatally shot eight office workers today after an argument in a central Melbourne post office building, then fought off security guards and jumped 11 stories to his death, police said.

Four of the dead were shot inside an elevator in the Queen Street building as the unidentified man stalked through several floors firing indiscriminately.

"It was a slaughterhouse. It was a gruesome sight, believe me," ambulance officer Ian Patrick said.

Five other persons were wounded, one seriously, in what police described as a 20-minute rampage.

Four months ago, a former military cadet went on a shooting spree after an argument with his girlfriend, killing seven persons and wounding 19 in a sniper-style massacre in the Melbourne suburb of Clifton Hill.

Police identified today's gunman only as a university student.

Five women and three men were killed. Although police said the gunman carried a "military-style rifle and ammunition," they would add only that he apparently had no criminal record.

The assailant, wearing a khaki jacket, white trousers and shirt, fired at will as office workers dived for cover beneath their desks, a sobbing secretary said outside the 18-story building.

"We have not been able to establish any particular motivating factor behind it," said Victoria state's assistant commissioner for crime, Vaughn Werner.

He denied initial reports that the gunman intended to rob a credit union on the building's fifth floor, saying the assailant had a "confrontation" with another man before he began shooting.

Tackled by security guards on the 11th floor, the man was disarmed but was able to smash a window and plunged headlong through the opening.

Since the Clifton Hill incident, Victoria has introduced Australia's strictest gun-control legislation, but within minutes of today's murders, irate radio listeners were demanding even tougher restrictions.