Parts of Sydney received their best rain in more than a year during an outbreak of severe thunderstorms in NSW on Thursday.

The storms developed as moisture-laden air fed into a trough of low pressure over the state's eastern districts.

Image: Thunderstorms over eastern NSW shortly before sunset on Thursday.

Some of the heaviest rain occurred in Sydney's northern and western suburbs. Mona Vale received 121mm and 66mm fell at Kings Langley. Terrey Hills registered 46mm of rain in half an hour and ended up with a total of 72mm in the gauge, their heaviest rain in 13 months.

The storms also produced very large hail in parts of Sydney. Golf ball-sized hailstones fell in Stanhope Gardens and Kellyville around 6pm, according to the Bureau of Meteorology. Similar sized hail was also reported around Penrith.

Elsewhere in the state, rain rates exceeded 40mm per hour near Goulburn during severe storms on Thursday afternoon. Flash flooding also affected parts of Wollongong.

More severe storms will develop over some central and northern districts of eastern NSW on Friday.

Visit http://www.weatherzone.com.au/warnings.jsp for the latest warnings.