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'Life-Threatening Flash Flooding' Forecast For Parts Of NSW

Up to 200mm of rain could fall in parts of the NSW Central Coast, creating potentially life-threatening flash flooding.

The Bureau of Meteorology (BOM) has forecast a very wet weekend for much of Australia's east coast, with parts of Queensland and NSW already copping a drenching on Friday.

Sydney and much of the northern NSW coast has already seen a torrential downpour as an intense coastal trough developed and moved south.

Greater Sydney is among the areas that could cop up to 200mm of rain overnight. Image: AAP

On Friday evening, the weather bureau said its models had predicted heavy overnight falls for central coastal areas of the state, including Greater Sydney, the Central Coast, Illawarra, Southern Highlands and the Blue Mountains.

It advised people in those areas to delay all non-essential travel.

"Overnight widespread torrential rain of 150 – 200mm is possible. This rain may generate areas of potentially life-threatening flash flooding," BOM said.

The development of very heavy rain through the Sydney Metropolitan was expected to begin early on Saturday morning, the bureau said.

Up to 300 millimetres has been recorded in some parts of the north coast with similar totals expected in the Sydney basin by the end of the weekend.

The severe weather crippled Sydney's rail network at the end of the PM peak after urgent signal equipment repairs were needed at Lidcombe.

Passengers on the T1 North Shore & Western LIne, T2 Inner West & Leppington Line, T3 Bankstown Line, T7 Olympic Park Line, T8 Airport & South Line, T9 Northern Line and Blue Mountains Line were  all advised to delay their travel.

The equipment has since been repaired but Transport for NSW advised commuters to keep allowing extra travel time as major delays continued into the night.

BOM acting NSW manager Jane Golding said rain could cause roads to be flooded in the city, but much less will fall in the state's drought-ravaged west.

"The bulk of the rain does look like it will affect the coastal ranges and the eastern side of the divide," Golding told reporters on Friday.

Earlier on Friday, BOM issued a severe weather warning for heavy rain and damaging winds stretching along the NSW coast, from the south of Wollongong up to Lismore.

A flood watch from the Brunswick to the Bega Rivers also remains in place.

In the last 24 hours, the bureau said some areas of the state recorded almost 300mm of rain, with stretches of the coast copping 150-200mm.

Byron Bay was one of the worst-hit areas, receiving more than 280mm  -- the highest total rainfall for that period -- while Coffs Harbour recorded 250mm.

In Sydney, heavy rain and pockets of flash flooding caused delays on multiple train lines and forced several road closures.

The NSW State Emergency Service has responded to 683 jobs across the state since the wet weather arrived on Wednesday.

NSW SES Commissioner Carlene York said most of the call-outs were for leaking roofs, damaged properties and stranded motorists -- some who had attempted to drive through flooded areas.

A car was washed off the road in the Northern Rivers region, where two people also had to be rescued from a caravan due to rising floodwaters.

Sydneysiders faced a wet commute to work on Friday. Image: AAP

York urged motorists to exercise caution and not attempt to drive through floodwaters.

When you see the water don't expect you can get through that water safely.

NSW Emergency Services Minister David Elliott also warned people to use their common sense.

"You would not walk into a bushfire so why would you drive through a flood?" he said.

Byron Bay copped widespread flash flooding on Friday. Image: Network 10

The NSW SES is working to assist people in metropolitan areas and will deploy resources in the Illawarra region as the weather system moves south.

The service will be using the NSW Rural Fire Service's State Operations Centre to manage incidents in the coming days.

Earlier, the RFS welcomed the heavy rain in bushfire-ravaged parts of the state.

"So nice listening to rain falling last night and driving with windscreen wipers on for a change this morning," RFS Commissioner Shane Fitzsimmons tweeted.

"BOM indicates continued rain this next week and importantly, falls across our fire areas which will be so welcomed by our farmers, firefighters and all affected."

The NSW RFS continues to manage more than 40 fires burning across the state.

The coastal trough is expected to remain over Sydney, the Central Coast and the South Coast on the weekend, with falls expected in Sydney, New Hampshire and Wollongong.

"What we are broadly expecting over the next couple of days is this coastal trough to hang around, to keep delivering this prolonged, steady rainfall," the weather bureau's Acting NSW State Manager Jane Golding, said.

Water overflows the banks of the Parramatta River after heavy rains in Sydney. Image: AAP

Golding said there was potential for the system to evolve into an east coast low in the next two days which could prove extremely dangerous.

"Bridges have been washed away … there are some really awful events that have happened from east coast lows," she said.

Most of the state's beaches will also be closed over the weekend, with king tides expected.

Minister Elliot warned those planning a beach trip should not expect to see any lifesavers.

Byron Bay locals wade through floodwaters. Image: 10 News First

Queensland

Queensland is also experiencing a drenching, with a 1000 kilometre flood warning now in place from Sydney to Brisbane.

The bureau reported steady rainfall overnight across Brisbane and the Gold Coast, with 170mm falling at Wappa Dam on the Sunshine Coast -- the highest total rainfall in the past 24 hours. Just over 90mm fell in one hour.

The weather bureau predicts another 100-300mm of rain will fall across northeast NSW and southeast Queensland over the next eight days.

With AAP