Heatwave for NSW ahead of Australia Day

Parts of NSW are in for a sweltering weekend ahead of Australia Day, with temperatures expected to reach 15 degrees above average in some places.

A high-pressure system over the Tasman Sea will prompt northerly winds distributing heat throughout the state, the Bureau of Meteorology says.

The mercury will rise from Thursday, meteorologist Melody Sturm said, with the Illawarra, south coast and western Sydney hit hardest.

"Especially Sunday and Monday, we'll see the highest temperatures ... with Bega being the highest," Ms Sturm said.

"It will actually be 42 degrees on Monday ... 14 to 15 degrees above average."

Further north in the Illawarra, Nowra will reach 39 degrees, 11 degrees above average, and Bowral will hit 37 degrees.

While coastal parts of Sydney will largely be spared from the hot weather, western and southwestern Sydney will not be so fortunate.

"Especially Penrith, we'll see temperatures of 41 on Monday, which is 10 degrees above the January average," Ms Sturm said.

"If you're looking more towards the coastal suburbs, we do see temps ranging into the upper 20s (to) lower 30s, which is a few degrees above average."

In the state's northwest, thunderstorms and dry conditions will cause the fire danger rating to jump to high.

A warning for damaging waves is also current along the NSW coast and Lord Howe Island.