By 8pm, ACT SES crews had been inundated with more than 150 requests for help, mostly for fallen trees, leaking roofs and minor flooding., and were expecting to work through the night on the clean-up.

In Canberra's biggest spray since February, the rain started at 3am and continued well into the evening.

Weather bureau data shows Canberra was hit by almost 50 millimetres of rain between midnight and 7:30pm, falling short of February 25th's deluge of about 66mm which saw widespread flooding in the capital.

The bureau issued a warning at 4:45pm as severe storms broke overhead in the eastern half of NSW and motorists were urged to take extreme care on flooded roads.

Bureau meteorologist Anita Pyne said thunderstorms could dump a lot of rain in a short amount of time.

While the bureau was predicting more rain for Friday and a possibly rainy Christmas, conditions were expected to ease into the weekend.

East of Canberra, at the South Coast, seas of up to two metres were recorded as southbound winds brought in rain.

However, conditions at the coast were expected to ease alongside those in Canberra, with mostly sunny conditions forecast for Sunday.

The mountains to Canberra's south, including those in Kosciuszko National Park, also saw storms on Thursday.

"Mountains are very good at triggering thunderstorms," Ms Pyne said.

"They can get winds going into the mountains that have nowhere to go but up."