Thousands of homes were left without power and a number of major roads flooded as a severe thunderstorm bore down on Sydney on Thursday, bringing with it heavy rainfall, damaging winds and the threat of large hailstones.

The Sydney storm cell was part of a long line of severe thunderstorms that moved through the Sydney basin, Hunter and Illawarra.

A spokeswoman for the weather bureau said the city looked like it had been hit with one severe storm cell, while the Camden and Illawarra areas saw multiple cells which increased the risk of flash flooding in those areas.