It's Big Wednesday in Sydney, where large and powerful waves are pounding the coastline and giving experienced surfers a reason to grin.

The Bureau of Meteorology warned that surf conditions were expected to be hazardous for coastal activities such as rock fishing and swimming, as the swell that had been building throughout Tuesday peaked on Wednesday morning.

Waves of nearly two metres in "big, straight, solid sets" were reported at Bondi Beach on Wednesday morning, surfing website Swellnet reported.

Jacob Cronje, a senior meteorologist at Weatherzone, said the powerful surf was hitting the coast from Sydney up to the Newcastle area.

"The surf is being enhanced in the wake of a cold front that's moving away from the coast," he said.

"The system that we had over the weekend, those sustained south-easterly winds, are helping to enhance the surf. The winds today will start to tend off-shore and become weaker as a high-pressure system takes over.

"Conditions will definitely calm towards the afternoon. Having said that, from tomorrow we could see those conditions returning as winds will pick up ahead of a low-pressure system that's moving up the coast."

Mr Cronje said a strong wind warning had been issued for Thursday from Batemans Bay up to the Hunter.

"Although there's no surf warning out for tomorrow at this stage, those seas could increase upwards of 2.5 metres," he said.

It comes as Sydney shifts this week from summery conditions into a more wintry chill.

Richmond hit a low of 0.5 degrees and Camden a low of 2.3 degrees just after 6.30am on Wednesday, the Bureau of Meteorology said, while the city dropped to a low of 10.4 degrees just after 6am.

Wednesday is expected to be a mostly sunny day in the city with a top of 20 degrees, rising to 21 degrees with a few showers on Thursday. But the mercury is then forecast to drop to a maximum of 19 degrees with sunny conditions on Friday, and a top of just 17 degrees and a possible shower on Saturday.

Weatherzone is owned by Fairfax Media, publisher of this website.