SYDNEY, March 31 (Reuters) - Floodwaters filled the streets of...

SYDNEY, March 31 (Reuters) - Floodwaters filled the streets of a major Australian town of more than 25,000 people on Friday, as a storm system generated by the powerful cyclone that pummelled the nation's northeast three days ago swept down the coast with heavy rain.

Cyclone Debbie hit as a Category Four storm in the north of tropical Queensland state on Tuesday, smashing tourist resorts, bringing down power lines, flattening canefields and shutting down coal mines.

Authorities on Friday told almost 20,000 people to evacuate to higher ground as driving rain in hinterland and coastal areas either side of the Queensland state capital, Brisbane, swelled rivers to record heights across the region.

"We have everything happening, we've got people on rooftops, we've had people stuck in vans, it's a disaster, an utter disaster," New South Wales State Emergency Services Controller Ian Leckie told Australian Broadcasting Corporation radio.

The cyclone was downgraded to a tropical low depression on Wednesday and on Thursday it unleashed squalls with torrential rain across a 1,200-km (745-mile) stretch of Australia's east coast, before slowly moving out to sea on Friday.

No deaths were reported, but authorities feared fatalities after logging more than 100 flood rescues during the night. Gales and huge surf swells lashed the coast still around Cape Byron.

At Lismore, the rising Wilsons River overwhelmed a levee protecting the rural hub in the Northern Rivers region of New South Wales, home to at least 25,000 people, and floodwaters flowed downtown, filling streets and swamping dozens of shops.

"The CBD is being flooded and because it's now very dark and it's also windy, there are a lot of people who're scared as well," former Lismore mayor Jenny Dowell told the ABC.

Tens of thousands of people in the affected areas are without electricity.

In the cyclone-hit zone further north, military helicopters, ferries and planes on Thursday evacuated hundreds of holidaymakers from the resort islands along the world-famous Great Barrier Reef and the Whitsunday coast that bore the brunt of the storm with wind gusts stronger than 260 kph (160 mph).

In the Bowen Basin, the world's single largest source of coal used to make steel, major miners Glencore and BHP (BHP), said they were still assessing the extent of any disruption to shipments.

About 2,500 insurance claims have been filed but Queensland's top two insurers, Suncorp Group Ltd (SUN) and RACQ, said it was too early to put a dollar figure on the damage.

One female tourist was killed in a car crash on Monday that police said was due to wild weather as Cyclone Debbie approached.