Swimmers visiting Sydney's beaches on the weekend were shocked by unexpectedly cold water. While bathers and boardshorts are usually all you need for a comfortable swim at one of Sydney's beaches in December, this certainly wasn’t the case during the last 48 hours. The water temperature off the coast of Sydney was down to around 17 degrees on the weekend, with even cooler water circulating near the shoreline. The unseasonably cold water was the result of an upwelling event. Upwelling occurs when a prolonged spell of northeasterly winds causes a layer of warm surface water to move away from the shoreline, allowing cooler water to rise from the depths in its place. A buoy located a few kilometres off the coast of Sydney measured a near-surface water temperature of 16.9C early this morning. This is 4C below average for this time of year and about as cold as it gets in December. There were even reports that competitors in the Bondi to Bronte ocean swim on Sunday required treatment for hypothermia. Unfortunately for these swimmers, this year's race was like taking to the water in July or August wearing nothing but bathers.