Two people have drowned and another is missing as authorities in New South Wales warn of dangerous conditions in the water.

Paramedics have been called to about a dozen near-drownings in Sydney on Saturday.

Authorities say a 23-year-old man died after he jumped into a blowhole and was swept out to sea at Warriewood on Sydney's Northern Beaches.

Another 23-year-old man is also missing after he was washed off the rocks at North Curl Curl.

Meanwhile, a one-year-old boy has drowned in a backyard pool at Mulgoa in Sydney's west.

NSW Ambulance Service spokeswoman Jacqueline Levett says most of the beach incidents have been near rocks.

"We're advising people not to take unusually high risks around rocks, most of the incidents appear to be because of weather conditions," she said.

"We also have been advised by the Bureau of Meteorology that we're expecting a southerly change into Sunday morning making these water conditions even worse."