Just when you felt it was safe ...

OCEAN swimmers have had a rude shock with water temperatures dipping as low as 17 degrees this past week - comparable to an average winter's day.

The bracing temperatures have been creating headaches for ocean swim organisers with some participants suffering the initial signs of hypothermia, while regular dippers have been dusting off their wetsuits.

The culprit is northerly winds moving the warmer surface water away from the NSW coast and drawing deeper, colder water to the surface - a scientific phenomenon known as "upwelling".

It has been responsible for wildly fluctuating sea temperatures. Last Wednesday, the temperature rose to 22 degrees before dipping to 17 degrees on Friday. The average temperature for this time of year is around 21 degrees.

The conditions meant 70 swimmers suffered the first symptoms of hypothermia during Coogee's 2.4-kilometre Express Glass Island Challenge ocean swim a fortnight ago, after the water temperature dropped to 16.8 degrees.

"It was unseasonably cold," said event co-ordinator Lisa Bolton. "But in the 11 years the race has been held, this has not been the coldest."

One competitor, Andy, described the conditions on an ocean swimming blog: "Just after one of the last yellow buoys having rounded the island, the cold got to me, panic set in and all co-ordination just went out of the window."

Last week's Bondi to Bronte ocean swim was also hampered by the chill.

"This was the coldest race we've had in five years of holding it," race director Stephen Ford said.

"The water at this time of year is generally 19 degrees, but due to the north-east currents this dropped the temperature ... We have had strange climate conditions this year and this in turn affects the temperature of the water."

Thirty-seven ocean swims are expected to take place between now and June next year.

Sydney's largest ocean swim, the 2.5-kilometre Macquarie Big Swim in January, says on its website that competitors will be able to wear wetsuits if the water temperature is below 18 degrees.

Swimmers may be puzzled by the chilly sea temperatures but scientists and oceanographers say upwelling is a common phenomenon.

A Bureau of Meteorology senior scientist, Dr Gary Brassington, said the cold water had surfaced due to persistent northerly winds. "With the air and sea temperature contrast, swimmers will really notice the sudden drop in the ocean temperature," Dr Brassington said.

And the good news?

"As soon as there is a southerly change, the warmer water will return," bureau forecaster Michael Logan said.