Dunedin City Council water production manager Gerard McCombie said the warm weather yesterday attracted the first reservoir swimmers of the season at Ross Creek.
Divers and swimmers should stay away from the reservoir because it had a history of toxic algal blooms.
''It's not a good idea to be swimming in potentially algae-laden water because sometimes they can generate toxins that can be harmful to the body.''
Since the council had been maintaining the reservoir, the water level had fallen, resulting in a heightened risk of algal blooms, he said.
If people wanted a swim they should go to a beach, river or public pool, he said.
Senior Sergeant Craig Dinnissen, of Dunedin, said police were called to the reservoir yesterday and advised a group of youths against diving and swimming in the reservoir.
