Ocean swims at Coogee and Mollymook were cancelled as southerly winds whipped up large and dangerous surf on Sunday.

Swells on the NSW coast were up to three metres, and in a rare sight, the waves in Sydney Harbour were big enough to lure surfers to Shark Bay, off Nielsen Park.

Brett Dutschke, senior meteorologist from Weatherzone, which is owned by Fairfax Media, said southerly winds and low pressure systems over the Tasman were causing the big seas.

''It's a very slow-moving system, so expect the surf to only gradually drop off during the week,'' he said.

At Coogee, breaking waves were dumping sharply onto a bank about 25 metres off the beach, making it too dangerous to hold an ocean swim.

There have been more ocean swim cancellations and postponements than in any other season, event organisers said.

Mr Dutschke said the past two months had featured consistently strong southerly winds and low pressure systems forming off-shore, bringing surges up the coastline.

An unusually high number of warm wet days has turned Sydney into the tropics. It rained on 22 out of 31 days in March, the highest number since 1990.

''That's more than Darwin had, and normally it's the other way around,'' Mr Dutschke said.

The rain is forecast to continue until Thursday, but it is expected to clear in time for a picture-perfect Easter.

The holiday is forecast to be dry, with temperatures in the mid-20s for the long weekend.