Police were forced to close Hyde St and move people off roofs after thousands flocked to the annual keg party dressed as everything from Tarzan to a yellow submarine.
At least one roof collapsed under the weight of revellers and there were reports of several injuries.
The party started early - about 5am - Otago University Students Association events co-ordinator Koren Allpress said.
She said portable toilets, barbecues and water stations had been set up on the street, along with a St John first aid area.
OUSA and police were checking bags ensuring people complied with the ban on glass.