'900mm in days': Thousands flee as NSW facing further 'extraordinary' rain

Thousands are being evacuated from their homes Monday morning as relentless rain continues to pummel parts of NSW.

Authorities say thousands of homes on the Mid North Coast have been flooded while the NSW State Emergency Service says it could take months to restore outages including electricity, gas and water.

Floodwaters are expected to rise to major levels along the Hawkesbury in Sydney's northwest, inundating places such as Windsor, Pitt Town, North Richmond, Freemans Reach and Colo.

Evacuation orders have been issued in Western Sydney, with NSW SES urging residents in Eastern Creek to evacuate by 3am and in the northwestern suburb of Windsor by 9am Monday.

Parts of Penrith, Jamiestown, Cornwallis, Freemans Reach and Pitt Town are among the areas ordered to evacuate.

The Blue Mountains has been battered by the conditions also, with Katoomba recording 168mm in the previous 24 hours.

"This is an event which far exceeds what has occurred in the last 50 or 60 years," Premier Gladys Berejiklian told reporters on Monday, warning residents not to become complacent.

"Unfortunately we are not through the worst of it potentially. That is why we need to brace ourselves for what will be very difficult weeks and months of recovery."

She said 18,000 people across the state had been evacuated from the state so far, 15,000 of those on the Mid North Coast.

NSW SES Assistant Commissioner Nicole Hogan told ABC News Breakfast to expect further evacuations throughout the day.

The SES believes up to 54,000 could be evacuated from the Nepean area.

Flooding in parts of the Hawkesbury-Nepean River system has reached levels not seen since 1961.

Sydney is expected to receive up to 70mm on Monday.

Transport for NSW urged commuters to reconsider travelling to work on Monday, advising many to work from home where possible.

Region facing worst flooding in nearly 100 years

And the torrential rain is set to continue on the Mid North Coast, where Coffs Harbour could see up to 200mm on Monday, according to Bureau of Meteorology senior forecaster Adam Morgan.

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The region has been inundated with "extraordinary" rain in recent days, with close to 900mm in the 6 days up to 3am on Monday morning falling in some locations around Port Macquarie.

Superintendent Shane Cribb of the Mid North Coast Police District said thousands of homes had been flooded to varying extents.

"I have never seen rainfall like this. I have worked in a number of floods and this is the biggest one I have worked in in my career... it is extensive and serious," he told ABC News.

Residents in Kempsey CBD were evacuated overnight, with the banks of the Macleay River expected to burst its banks imminently.

Kempsey recorded 173 mm of rain between 9am Sunday to 4am.

Evacuation orders are also in place for low lying areas of Macksville, Port Macquarie, the lower Macleay, Wauchope and Rawdon Island, Taree and Wingham.

Emergency Services Minister David Elliott said the region was facing the worst flooding conditions since 1929.

Two storm bands to combine Monday

Authorities are warning residents the conditions will continue in the coming days as two weather systems are set to collide late on Monday.

A tropical low in northern WA and a coastal trough affecting NSW will merge, according to BOM.

The system will move south, with the NSW South Coast set for the worst of the rain on Tuesday, while northeast Tasmania will see up to 60mm on Wednesday.

Overnight, emergency workers conducted 150 more flood rescues and responded to 1500 calls for help, taking the total so far for the weekend so far to more than 10,000.

There are 20 evacuation orders in place running from the Mid-North Coast down to the Illawarra, including Sydney and western Sydney, with more expected to be declared on Monday.

Almost NSW 200 schools will be closed on Monday but with the extreme wet weather continuing, decisions will be made hourly on further closures. A list of affected schools can be found here on the Department of Education website.

Extensive outages could last months

In a recorded video, Prime Minister Scott Morrison said the floods were "very concerning" and thanked the "amazing " emergency services on the ground.

Hundreds of homes have been damaged, including one filmed floating down the Manning River in Taree, but Mr Elliott says it is only a matter of time until lives are lost too.

Video of one flood rescue was shared by NSW Fire and Rescue shows nine people, including four children, being saved from Tuncurry caravan park after it became engulfed by the floodwaters.

The Hawkesbury is predicted to reach peaks of up to 15 metres and the SES says homes and properties will be flooded, some up to roof height.

The floods will cut off evacuation routes and cause lasting outages to utilities, the SES said as it urged residents to prepare to evacuate.

"Extensive outages of water, electricity, sewerage, telecommunications and gas are expected to last many weeks or months," the SES said.

Head of Resilience NSW Shane Fitzsimmons told 2GB the state still has "days of this extraordinary event to roll out".

"Communities will be threatened and compromised for a while yet.”

Superintendent Cribb concurred, saying: "Unfortunately we have a long way to go".

With AAP

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