The mid-summer heatwave baking much of NSW has given parts of Sydney their hottest start to a year in at least decades, and may have as long as another week to run.

While outback towns including Bourke climbed into the mid-40s on Monday, Sydney's western suburbs weren't far behind. Penrith reached 43.2 degrees and Richmond 42.

The Bureau of Meteorology issued a fire weather warning for Monday for the Sydney, Hunter, southern ranges and central ranges regions. The NSW Rural Fire Service also issued total fire bans for those areas amid "severe" fire danger ratings for the day.

There is also thunderstorm activity in the city's west, with the bureau issuing a warning for a severe storm including the possibility of large hailstones.