Parts of Sydney received extremely heavy rainfall overnight with more than 100 mm recorded at a weather station in the city's north, while falls of more than 50 mm were recorded at locations from the northern suburbs of Wollongong up to the Central Coast.

In the 24 hours to 9 am Wednesday, Sydney's official weather station at Observatory Hill (near the southern end of the Sydney Harbour Bridge) received 83.4 mm, which made it the city's wettest day since February and the wettest October day in four years.

The heaviest recorded overnight fall in Sydney was 106 mm at Chatswood on the North Shore (including 78 mm in three hours). The SES unit in Mosman, just a few kilometres away on the lower north shore, reported multiple flood-related calls.

Marricvkville in the inner west came close to a triple-figure total with 99 mm.

Rainfall totals were generally lighter in Sydney's outer west, however many suburbs still recorded their heaviest rain since February.

Richmond in the city's far northwest received 21.6 mm, which was the first time it had exceeded 15 mm in a day since May, in what has been a very dry period for western Sydney.

What caused the sustained heavy downpour?

Weatherzone meteorologist Ben Domensino explains that Tuesday night's heavy rain was caused by a low pressure trough moving up the central NSW coast.

"This allowed moisture-laden winds to converge over parts of Sydney and the Central Coast," Domensino said.

Showers to continue for several days in Sydney

Relatively cool (for October) showery weather is likely to remain the dominant pattern in Sydney until at least the end of the week, with maximum temperatures expected to fall a degree or so below the October average of 22.2°C most days.

The mercury is expected to reach just 19°C this Wednesday, which would make it the coolest day of the month to date.

While cool southeasterly winds will pick up moisture from the Tasman Sea and push frequent showers onto Sydney's coastal fringe and suburbs further inland, no heavy daily totals of the type recorded overnight are expected.