SEVERE thunderstorm warnings have been issued for large parts of NSW including Sydney, the Blue Mountains and the Hawkesbury.

The warnings are for large hailstones, heavy rain and damaging wind.

The Bureau of Meteorology issued warnings just after 2pm, saying heavy rainfalls could lead to flash flooding in some areas over the next several hours.

The bureau said places affected included Scone, Penrith, Wollongong, Bowral, Orange, Mudgee, Bathurst, Katoomba, Dubbo, Parkes, Cobar and Bourke.

It said storms closer to Sydney were about to hit Kurrajong, Richmond, the Wollemi National Park, Castlereagh, Cranebrook, Londonderry, Colo, Wilberforce, Windsor and Riverstone.

The State Emergency Service advises people to move cars undercover and away from trees, secure loose items around houses, stay indoors and keep away from fallen power lines, creeks and storm drains.