Sydney and the Hunter region were battered by thunderstorms overnight with heavy rain continuing into the morning, causing flooding and damage.

Meteorologists said there were about 30,000 lightning strikes in the Hunter region overnight, with the area hit by "huge" hailstones reported to be about five centimetres in diameter.

The NSW State Emergency Service received 250 calls for assistance overnight, with the Hunter and Sydney metropolitan region the worst affected areas.

Sydney commuters face a slow trip into work on Friday morning with wet roads and flooding in parts of the city, including Drummoyne, Moore Park and Beaconsfield. Trains between Sydney and Newcastle were also affected due to power supply problems caused by the weather.