NSW rain to continue after Sydney drenched

Drivers are being urged to avoid flooded or debris-strewn roads and seek alternative routes.

The Bureau of Meteorology's radar showed heavy falls with thunderstorms with many suburbs recording up more than 100mm on Tuesday.

Many roads are flooded and ferry services have been cancelled between Parramatta and Sydney Olympic Park on Wednesday due to flooding and debris on the swollen Parramatta River.

The SES received 256 requests for help from 6pm on Tuesday until 8.30am on Wednesday, with the majority of those in the Sydney metropolitan area.

Marrickville in Sydney's inner west copped the heaviest falls, recording 169mm of rain in the 24 hours from 9am on Tuesday.

Sydney Airport recorded 137mm, Rose Bay in the inner east 121mm, and Canterbury in the south west 119mm.

More than 100mm was recorded in a number of locations around western Sydney including at Bankstown, Lidcombe, Auburn, Guildford, Toongabbie, North Ryde and Castle Hill.

On the Central Coast, Wyong recorded 160mm of rain and Kulnura 102mm.

The SES was involved in flood rescues at Schofields in western Sydney and Collaroy on the northern beaches.

In the north of the state, another rescue was conducted at Urliup, north of Murwillumbah near the border with Queensland.

BOM meteorologist Bradley Wood told ABC radio on Wednesday the intensity of the rain had eased.

"But we are still seeing persistent showers coming across the coast, and they'll continue much of today and really for the foreseeable future ," Mr Wood said.

South of Sydney in the Illawarra, bureau warns "a constant feed of thunderstorm activity" is likely to produce heavy rain on Wednesday.

A slow-moving trough is expected to produce "heavy to intense" rain and the possibility of severe weather in the NSW northern rivers, which could cause flash flooding.

The drenching comes after NSW experienced a wetter than usual summer in the midst of a La Nina weather pattern that's predicted to stretch into autumn.