Cool conditions and a strong southerly swell of as much as four metres had swimmers, boaties and rock fishers heading for dry ground on Sunday.
Westerly winds increased to 45 to 60 km/hr during the day and the Bureau of Meteorology issued a gale warning for coastal areas stretching from the Hunter down to the Victorian border.
Large and powerful surf conditions pounded the coast and the Bureau cautioned against swimming, surfing, crossing bars by boat, and rock fishing.
Surf Life Saving NSW said the southerly swell was expected to reach four metres at its peak late Sunday afternoon, with a swell period of around 12 to 13 seconds.
“With the onset of these dangerous condition we strongly encourage swimmers to follow all safety warnings and avoid entering the water unless there is an active patrol on duty, which will be signified by the red and yellow flags," Surf Life Saving NSW Lifesaving Manager Andy Kent said.
“While the official patrol season has ended there are still some locally patrolled beaches in metro areas and I would not encourage anybody to swim in an un-patrolled area during this time frame,” he said.
Winds were expected to decrease to 15 to 25 km/h late Sunday evening.
Sam Brown, a meteorologist at Weatherzone, said the size of the swell was more likely to be 1-2 metres by the time the waves reached the shore.
For more experienced surfers, conditions may provide good rides for Sunday and Monday.
"The whole day will actually be very good for surfing," Mr Brown said, referring to Monday.
Sydney's relatively mild autumn came to an abrupt halt on Saturday with a powerful cold front sweeping across southeastern Australia sending the mercury plummeting.
Saturday's maximum in Sydney was only 15.7 degrees yesterday, its coldest day this early in the season in 31 years. Canberra peaked at 9.1 degrees to also record its coldest day this early in the season in 31 years, Rob Sharpe from Weatherzone said.
The cold snap also brought snowfalls to resorts at Thredbo and Perisher and even as far north as Orange.
Overnight temperatures will remain low in many inland regions of the state but Sydney can look forward to a generally mild and dry week with only a few showers along the coastal region, Mr Brown said.
Weatherzone is owned by Fairfax Media, publisher of this website.