These conditions are driven by a low pressure system, also referred to as an East Coast Low, which is forecast to develop off the southern New South Wales coast later today.

Bureau of Meteorology acting New South Wales regional director Stephen Lellyett said the bureau had issued a severe weather warning earlier today and will issue further warnings as the situation evolves.

“The focus is currently on the Illawarra district and northern parts of the South Coast, with heavy rain and the potential for flash flooding being the main threats,” Mr Lellyett said.

Rainfall totals in the range of 100–200mm over the duration of this event are forecast, with higher localised falls of over 300mm possible.

A flood watch has been issued for the Illawarra, South Coast, Lachlan and Nepean River Valleys.

“The East Coast Low is expected to weaken considerably overnight Tuesday, with lighter rainfall expected on Wednesday," he said.

Mr Lellyett said Sydney would be on the fringe of this weather system.

NSW SES Commissioner Adam Dent is urging people to plan ahead for the heavy rainfall forecast.

“In the next 48 hours, particularly in the Illawarra, we could see flash flooding closing roads and access routes. People in the Illawarra need to plan with their family travel arrangements, including school pickups, in case they are delayed during these conditions,” Commissioner Dent said.

“We don’t want people driving through floodwater and risking their lives and the lives of our volunteers."