Ex-Cyclone Debbie's perfect storm stretches more than 1000km to NSW

New South Wales has started feeling the effects of ex-tropical Cyclone Debbie which has stretched 1200kms to the south and there are now fears a second state will cop the storm's wild winds and flash flooding.

Debbie caused mass devastation to far north Queensland, with people caught in rising waters as the deluge spreads south, and thousands have been left without power.

The cyclone was downgraded to a tropical low depression on Wednesday but on Thursday was driving squalls with torrential rain across a more than 1000km stretch of Australia's east coast, swelling rivers, causing flash floods and prompting authorities to tell 40,000 people to evacuate.

More than 800 Queensland state schools and day care centres were closed on Thursday in an unprecedented decision to keep the roads clear. They will remain shut on Friday.

Emergency services urged businesses to let staff go home early and told residents to stay indoors and off the roads. Inland areas which haven't seen rain in months are now at risk of flash flooding.

Residents of 71 homes in southeast Queensland's Lockyer Valley are being urged to keep a close eye on rising waters with authorities warning low-lying areas may flood overnight.

Energex reported tens of thousands of Queenslanders are without power in areas across the Sunshine Coast, Brisbane, Moreton Bay, Logan, while there are reports power is out at some Sydney suburbs including Paddington and Thornleigh.

• LATEST: Lismore among NSW towns evacuated, as QLD schools stay closed

• 'Stay out of floodwater': Bull shark washes up in north Queensland after Cyclone Debbie

• 'Leave town': Looters strike Whitsundays restaurant amid Debbie's destruction

Four houses at Queensland's Laidley and 67 in Forest Hill could be inundated in the same region where 22 people were killed when flash floods ripped through in 2011.

A police spokesman confirmed there was no evacuation order for the area but said people were being warned to prepare to leave and stay with relatives by the local disaster management group on Thursday night.

Not quite a cyclone, but its low-pressure remnants are colliding with a cold front moving up from the south, bringing torrential rain and flooding to south-east Queensland and NSW.

"This severe weather system that began with Cyclone Debbie and is tracking down the coast is causing havoc across our state," Queensland Premier Annastacia Palaszczuk told reporters in Brisbane.

The rain band is moving south east from Mackay in Queensland's north, across the Central tablelands, south to the Sunshine Coast, Brisbane, Gold Coast and into the NSW Border.

The State Emergency Service has issued a Flood Evacuation Order for tens of thousands of NSW Northern Rivers residents as rising water from heavy rain threatens homes in the district.

In Lismore in the north of NSW state, the SES ordered 7000 residents in low-lying areas to leave after forecasts predicted the town's worst flood in nearly 20 years.

Lismore south, north, Lismore CBD, Chinderah, Kingscliff, Fingal Head, Bilambi, Murwillumbah, Condong and Tumbulgum were all ordered to evacuate on Thursday afternoon.

Residents have been told they must leave immediately, with additional warnings in place for the Tweed Heads region, in northern NSW.

A levee protects the rural hub in the Northern Rivers region of New South Wales, home to at least 25,000 people, but most of those downtown planned to seek higher ground, Geoff Baxter, a barman at the Richmond Hotel said.

"We're clearing out the pub, mate, and closing it up. All the shops got closed, everyone's clearing up their cellars," he said.

Flood warnings extended from the Tweed to Coffs Harbour, with up to 400mm of rain predicted.

Lismore, Grafton, Coffs Harbour, Glen Innes and Inverell were all hard hit - not just by flooding, but wind with gusts up to 125 km/h.

Murwillumbah residents were caught by surprise, telling 7 News they had never seen the Tweed River rise this quickly before.

More than 250 millimetres of rainfall is expected to come, and some areas could be hit with isolated falls of more than 400mm.

The storm is expected to dump 45mm of rain on Sydney.

An extra 50 SES personnel have been sent north, along with fire brigade swiftwater rescue teams.

Prime Minister Malcolm Turnbull and Opposition Leader Bill Shorten have also flown in to help with the clean up at hard-hit Bowen, in northern Queensland.

The PM has also promised to fast track disaster relief funding for what's going to be billions of dollars in damage.