Cyclone Debbie has been downgraded to the weakest category overnight, as Queenslanders are set to wake up to a huge clean up following the "monster" storm.

Last night Queensland Premier Annastacia Palaszczuk predicted "shock and awe" in the state when the full extent of the devastation wrought by the cyclone is revealed.

At midnight, the Bureau of Meterorology downgraded Debbie to a category one cyclone, with winds near the centre at 85km/h and wind gusts of up to 120 km/h as it passed by Collinsville and headed inland.

Cyclone Debbie was forecast it to weaken further to a tropical low in the early morning, but heavy rain in the region throughout today was still expected.

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At its most ferocious the cyclone downed trees, stripped buildings and left shorelines swamped after making landfall as a category four storm at midday yesterday near Airlie Beach.

Trinity Turner posted this image to the Sarina Community Noticeboard showing sea foam whipped up from Cyclone Debbie in Queensland. Photo / Facebook
Trinity Turner posted this image to the Sarina Community Noticeboard showing sea foam whipped up from Cyclone Debbie in Queensland. Photo / Facebook

The storm was downgraded to a category three cyclone just after 3pm, but strong winds continued to lash Airlie Beach, Proserpine and Bowen late in the afternoon.

The state's premier and police commissioner issued grave warnings to residents.

The premier, who on Monday warned it would be a "monster", said the state would be dealing with the impact of the "scary" cyclone for the next three to five days as it moved down the coast.

"I think there is going to be a lot of shock and awe in the morning," she told Ten News yesterday.

She said rapid assessment teams would be sent to observe the damage at first light today, later revealing even she was "bracing" for Debbie's full impact being unveiled.

A man, believed to be aged in his 60s, was badly injured when a wall collapsed on him in Proserpine.

He is now in a stable condition in Proserpine Hospital.

"We are going to get lots of reports of damage and sadly I think we will also receive more reports of injuries, if not deaths. We need to be prepared for that," Commissioner Ian Stewart said yesterday afternoon.

The Australian Defence Force has mobilised soldiers, vehicles, aircraft and other resources to respond to Debbie, which the Insurance Council of Australia has already declared a "catastrophe".

The storm earlier pounded the Whitsunday Islands, with gusts of 263 km/h recorded at Hamilton Island while the jetty at Daydream Island was virtually washed away.

More than 48,000 homes were without power across the Bowen, Mackay and Whitsunday regions and more than 400 schools and childcare centres closed.

Forecasters are also warning people to remain vigilant about damaging deluges following the cyclone, with significant rain expected down the coast towards the state's southeast.

- AAP