Emergency services have responded to hundreds of calls for help after rain bucketed down on Sydney and surrounding areas at night.
About 300 calls were received by the NSW State Emergency Service on Sunday night and Monday morning, mostly from Sydney's storm-hit northern beaches and western suburbs.
Call-outs included requests to deal with downed branches and leaking rooftops, including in the north-western suburb Castle Hill where a large tree fell on a house.
Authorities warned slow-moving showers and thunderstorms forecast for Monday and Tuesday had the potential to cause flash flooding in parts of the state's mid-north coast and Northern Rivers, as well as the far west.
Much of the predicted rain could fall on popular holiday spots, triggering an SES warning for people camping in low-lying areas.
"Please be safe when camping or caravanning near rivers, as flash flooding can occur very quickly and without warning," assistant commissioner Nicole Hogan said.
Ballina, in the Northern Rivers region, had received nearly 100mm of rain between 9am and mid-afternoon on Monday.
Meanwhile, Kurrajong Heights, in the Blue Mountains, recorded one of the state's highest rainfall totals after receiving 151mm in the 24 hours to Monday morning.
There were also significant falls in Sydney's west, including 122mm at North Rocks and 103mm at North Parramatta.
Thunderstorms were possible across an area from north of Newcastle to the Queensland border on Monday afternoon and evening, as well for much of the state's far west.
The same area was facing the chance of further rainfall on Tuesday and the potential for already soaked catchments to overflow.
