Police said the scene has been contained, and there was no ongoing threat to public safety.

On Wednesday, film student Aditya Jain was cooking his dinner on the sixth floor of Cooper Lodge when an alarm sounded to evacuate after 7pm. Others described running out the door without their shoes, or with few belongings, as the alarm system advised them to "avoid the fourth floor".

Students were put up in the nearby refectory with pizza and movies until about midnight, when most were allowed back in, save for those on the fourth floor.

But late on Wednesday morning students were evacuated again and more emergency crews with hazmat gear descended on the campus. By 8pm mostly police remained on scene.

Third-year law student Mudit Bhandari said he thought the initial evacuation was a hoax at first.

The next morning when he returned to the lodge after class about 11.30am he found even more police and fire trucks outside.

"They wouldn't let us back in, it was all a big quarantine zone," he said.

While level three and four of Cooper Lodge, as well as commercial buildings nearby, were evacuated again on Wednesday afternoon, by the evening a university spokeswoman confirmed just parts of the fourth floor remained cordoned off.

A small number of affected students had been relocated to different rooms for the night, she said.

Melodie van Wyk said she and her friends had remained inside the lodge during the evacuation as they were worried if they left they would be locked out.

"We're kind of trapped here until they tell us what's happening, we need to study," she said.

Mr Bhandari said the university had been looking after students well as they waited it out.

Third-year student Harrison Thiele, who lives on campus nearby, said fire alarms had often gone off in Cooper Lodge but he hadn't heard of anything like Tuesday night's incident before.

"My friend was on the first floor, he didn't even have time to get his shoes on."

Students travelled to neighbouring halls or pulled out sleeping bags in the Refectory to wait out the initial evening evacuation, he said.

“They’ve said they’ll give us extensions on work and there’s free food and counselling, there's support," he said.

The university's director of student life, Scott Pearsall, thanked students for their patience in an email on Thursday morning.

"We understand that disruptions can be particularly difficult at this time of the year," Mr Pearsall's email read. "Our student welfare and medical and counselling teams are available for support."

The university's student representative council postponed its Halloween event on Wednesday night due to the partial closure of Cooper Lodge.