Record temperatures are possible across much of NSW on Saturday, elevating bushfire risks to "severe" in much of NSW, and increasing the health risks for those playing sport.

The NSW Rural Fire Service has issued total fire bans for 12 of the state's 21 districts on Saturday, including the Sydney, Newcastle and Illawarra areas. Similar restrictions may be extended into Sunday.

For Sydney, the forecast top of 33 degrees for both Saturday and Sunday would be about 13 degrees above the norm for September.

A region of "quite enhanced high pressure" over the interior of Australia had allowed heat to build up, Dean Sgarbossa, a senior forecaster on the Bureau of Meteorology's extreme weather desk, said.