The Bureau of Meteorology is warning of severe thunderstorms producing heavy, locally intense rainfall over much of the state.

The warning zone stretches for almost the entire state - from north of Narrabri in the north west slopes and plains region to Albury at the Victorian border - and covers all of the ACT.

Only the state's far west will be spared the drenching.

Lismore in the Northern Rivers has been among the hardest hit locations on Wednesday, copping more than 120mm of rain in three hours on Wednesday afternoon, with the SES performing five flood rescues in just over an hour.

At Lismore 199mm fell between 9am and 5pm, bested only by Tuckurimba to the south, which got 217mm.

The BOM is asking residents to avoid the Lismore CBD due to flash flooding. The Wilsons River has seen minor flooding, with heights of 6.73m, and moderate flooding to a height of 7.2m is possible on Wednesday evening.

Northern Rivers residents are likely through the worst of the thunderstorms, but will still endure heavy rainfall on Wednesday evening.

The Southern Highlands and Greater Wollongong regions were hit with severe thunderstorms on Wednesday, which will probably resume, with warnings of flash flooding. Moss Vale received 42mm of rain in just one hour.

The rain has led to alarm bells for flooding throughout the state.

There's a major flood warning for Coutts Crossing near Grafton on the Orara River, with the BOM warning the river could reach 12m on Wednesday evening. The Tweed River has a moderate flood warning, with water levels set to rise at Murwillumbah overnight.

There are also flood warnings at Grafton, Ulmarra and Maclean, where waters will continue to rise on Thursday morning. Coraki, Bungawalbin and Woodburn may see flooding on Wednesday evening. Thora and Kempsey have also faced flooding.

Flood warnings elsewhere have been downgraded after waters hit their peaks.

The weather has caused havoc across the state in the past few days.

Rock slides, bridge closures and even a yawning sinkhole in a residential Coffs Harbour street have been caused by the extreme weather event.

A number of roads across the affected regions have been closed, and authorities are urging drivers to be careful.

The SES attended more than 300 jobs overnight and has performed more than 30 flood rescues since the extreme weather began on the weekend.

Since the downpours began on Saturday, the SES has attended more than 1900 jobs.

"The majority of those jobs are fixing leaking roofs and damage due to heavy rainfall or trees down due to strong winds and we're also getting requests for sandbags," an SES spokeswoman said.

The SES has lifted evacuation orders for Condong and Tumbulgum, near where the Tweed River burst its banks on Tuesday, and North Macksville and Murwillumbah.

Australian Associated Press