Warragamba Dam has begun spilling over and residents of a town south of Sydney are preparing to evacuate as rivers rise, bridges close and roads flood across NSW.

Residents along low-lying areas near the Nepean River were warned the dam had started spilling about 4.20am on Friday after rainfall totals of up to 147mm in the 24 hours to 5am on Friday across the Hawkesbury-Nepean Valley.

Helicopter footage shows a NSW State Emergency Service volunteer checking the car, which was almost entirely submerged on the Menangle Bridge.CREDIT:NINE NEWS

A warning is in place for moderate flooding in the Upper Nepean. The Menangle Bridge has closed as floodwater rushed across the road, forcing a motorist to leave their car as it became trapped on the bridge.

Helicopter footage showed a NSW State Emergency Service volunteer checking the car, which was almost entirely submerged.

The Nepean River at Penrith is likely to reach the minor flood level by midday, and the Hawkesbury River at Windsor and North Richmond could reach moderate flood levels by Saturday.

The overnight deluge led to flash flooding in Picton, to the south-west of Sydney, where NSW SES volunteers were doorknocking residents in the early hours of Friday to prepare people to evacuate.

The Warragamba Dam started spilling at 4.20am. CREDIT:NICK MOIR

Argyle Bridge, which is the main access road for the centre of town, may close and cut off residents as the Stonequarry Creek rises.

The SES had a busy night in areas south of Sydney, which made up the lion’s share of the 447 storm-related incidents in the past 24 hours. There were 291 incidents in the Illawarra and South Coast area, 86 in the Sydney metro area and 12 in the Hunter and Central Coast area.

Most of the 49 flood rescues were in the Illawarra and South Coast areas and in Picton and were mostly people trapped in their cars on flooded roads.

The NSW SES is warning motorists to stay safe on the roads as the long weekend gets under way.

A car went underwater on the Menangle Bridge. CREDIT:KATE GERAGHTY

A severe weather warning remains in place for heavy rainfall and flash flooding for the Illawarra with six-hourly rainfall totals most likely between 40 and 60mm and isolated totals of 100mm possible.

Areas of particular concern for flash flooding are Wollongong, Nowra, Bowral, Campbelltown, Huskisson and Ulladulla.

It comes after parts of NSW were drenched overnight. Cringila, a suburb south of Wollongong, copped 206mm of rain in 19 hours. Sydneysiders were startled by huge thunderclaps and bright lightning strikes on Thursday night and experienced a soggy 57mm in 19 hours.

“Even though the rain is forecast to ease later on today, any further wet weather in the next few hours and for the rest of the day will be falling onto saturated land, particularly around the Illawarra, adding to or extending potential flooding impacts,” senior meteorologist at the weather bureau Angus Hines said.

“There have already been flash flooding impacts with closed roads and inundated homes, and this could continue or even expand with the further rainfall, particularly through the morning hours.”

Flash flooding is also affecting the transport network with buses replacing trains on the South Coast Line between Wollongong and Port Kembla, and between Thirroul and Waterfall. On the Southern Highlands Line, buses are replacing trains between Campbelltown and Moss Vale.