Wild weather including rainfall in excess of 100mm is expected to hit parts of NSW during Wednesday's morning commute, with police urging motorists to stay off the roads if possible.

The driver of the looming big wet is a large low-pressure system that is developing over the state and is predicted to cross the coast between Sydney and Wollongong in the early hours of tomorrow.

From about sunrise on Wednesday, rain is expected to drench the Sydney basin, especially in the south, and the Illawarra. Credit:Nick Moir

"Wherever does see the heaviest falls could see well in excess of their typical November rainfall, just in that period from late tomorrow morning through to tomorrow afternoon," Bureau of Meteorology spokesman Simon Louis said.

The bureau is forecasting 70-100 millimetres to fall on Sydney's Observatory Hill during Wednesday, with Campbelltown looking at 100-150 millimetres and Penrith 70-100 millimetres, according to its updated predictions as of Tuesday afternoon.