A low pressure system will wreak havoc over Sydney on Monday with high rainfall and possible severe thunderstorms forecast for parts of New South Wales .

The east coast of Sydney is expected to receive between 20 and 30 millimetres of rain, although rain will be less heavy in the west, with about 15 to 25 millimetres forecast for Penrith.

A low pressure system will wreak havoc over Sydney on Monday. Credit:Nick Moir

Senior forecaster at the Bureau of Meteorology, Mohammed Nabi said it will be a wet morning for commuters in Sydney with rain increasing throughout the day.

"It becomes a bit more prevalent during the day, people get into work and it will just seem like it's raining even more as opposed to easing off," he said.