Sydney is in for a hot few days but the mercury should ease back to milder conditions on Christmas Day.

The heatwave now searing south-eastern Australia will spill into the Sydney Basin from Saturday, with temperatures expected to reach 30 degrees and 32 on Sunday.

Participants dressed as santa take part in a world record attempt for the world's largest surf lesson at Bondi Beach. Photo: Getty Images

Inland regions, such as Penrith, will cop a maximum of 37 degrees on Saturday and 39 degrees on Sunday before Monday's top tapers off to 33 degrees, the Bureau of Meteorology says.

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"Residents of western suburbs will find a lot of relief by heading to the coast," Guy Dixon, a meteorologist with Weatherzone, said.

Fire weather conditions are severe across parts of South Australia and Victoria on Friday as a heatwave may break early-summer records. Adelaide, for instance, is predicted to reach 40 degrees for four consecutive days by Saturday - an event not recorded this side of January 1 in almost 130 years of data.

The beach will be the place to head this weekend - if not on Christmas Day. Photo: Getty Images

The heat, though, should subside for Sydney for the run-up to Christmas, with most days reaching the low to mid-20s in the city.

On the current bureau forecasts, Sydney's top will be 26 degrees on Christmas Day, on a partly cloudy day with a possible shower. Similar conditions are likely in inland regions such as Penrith or Liverpool, with temperatures reaching a couple of degrees warmer.

North-easterly winds will prevail for most of the week, including on Christmas Day. Those winds will tend to push up cooler waters so that temperatures in the sea may be on the brisk side.

"Christmas Day will fall midway through [another] warming trend with the peak of the heat likely on Boxing Day," Mr Dixon said.

Those travelling inland, particularly to Victoria, should keep a watch out for fire warnings. Along with the current heatwave, an extended dry spell means vegetation is primed for fires, particularly in the state's west, Mr Dixon said.

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