Dunedin police have sounded the alarm about suspicious people and known criminals lurking around the University area, preying on vulnerable students flatting for the first time.

Senior Sergeant Craig Dinnissen, of Dunedin, said people known to police were seen hanging around North Dunedin streets near the University of Otago last night, and warned it only takes a few minutes to burgle a flat.

"Last night there was clearly non-students up in that part of town ... they're faces we know.''

Students were urged to keep their flats locked at all times, even if popping out a brief period, and to be aware of what their flatmates were up to.

"You can't afford to leave your door unlocked because your [flatmate] is coming back in half an hour.

''If people see an opportunity to exploit the vulnerable, they will.''

Police would be conducting patrols on foot around the University of Otago area and speaking to students about keeping safe, he said.

''If we can work with them to ensure it's a safe environment to party, so windows don't get smashed, people don't get cut feet or arrested, that's what we want to do.''

Police did not want to see a return to the days of the "Undie 500'' riots of a decade ago, where a University of Canterbury Engineering Society car rally convoy from Christchurch to Dunedin would frequently result in massive riots.

In 2007, drunken rioting led to 69 arrests from a crowd of more than 1000.

Two years later a mob of hundreds advanced on a line of riot police and pelted them with bottles.

At the time, an officer said police used so much pepper spray to subdue the crowd they had run out, and had to call for resupply.

Snr Sgt Dinnissen said police did not want to see a return of those scenes.

Last night, police were called after two reports of people throwing bottles resulting in smashed windows, but officers were unable to identify the culprits.