Australia braced on Saturday for heavy rains and floods after weather authorities downgraded two cyclones that lashed its north and northeast, damaging homes and snapping power links, but there were no reports of casualties.

Troops were on standby to help with clean-up efforts in the northeastern state of Queensland after heavy rains and winds in excess of 200 kph (125 mph) brought by Cyclone Marcia on Friday.

Weather authorities warned of destructive winds, heavy rains and abnormally high tides as rivers swelled, but officials said the storm's impact had been weaker than expected.

"At this stage, everyone is breathing a deep sigh of relief that there has been no loss of life," Queensland Premier Annastacia Palaszczuk told reporters in state capital Brisbane.

However, most properties in the northern Queensland towns of Rockhampton and Yeppoon had been left without power, she added.

The worst of the rain would pass by Sunday as the storm is moving quickly back out to sea, the weather bureau said.

In Rockhampton, long queues snaked out of the only fuel station left with the power to run its petrol pumps, as people flocked to refill cars and get fuel to operate generators.

Residents ducked fallen trees and power lines to gather at the few automatic cash machines still functioning.

Heavy machinery was pressed into assist efforts to clear debris on the road to Yeppoon, the coastal town hit hardest by Marcia, but fallen power lines left many roads impassable.

The fury of the cyclone drove Demelza and Shaun Bischoff out of their home, along with their three children.

"It felt like the whole house was going to crumble," Demelza Bischoff told Reuters, describing how storm damage had forced the family to move from room to room of their home.

"It started getting really bad, the tin roof started lifting and the ceiling blew out," she said. "Then we took shelter under the kitchen table with mattresses all around us."

Now back home, they put furniture and clothing in their garden to dry out in the hot sun.

In northern Australia, teams were heading to two communities on the remote islands of Goulburn and Elcho to weigh up damage after Cyclone Lam, packing wind speeds ranging up to 240 kph, began to lash the Northern Territory late on Thursday.

The weather bureau has also downgraded that storm, to a tropical low moving in a south-westerly direction, with warnings of heavy rainfall, flash flooding and some storm surges.