Queensland is bracing for "monster" Cyclone Debbie.

Queenslanders are being warned to brace for “a monster” as intensifying Cyclone Debbie continues to thunder towards the state’s northern coastline.

Debbie is expected to hit the coast as a Category 4 tropical cyclone just south of Bowen at 9am on Tuesday, ahead of high tide at 9.44am.

The impending storm has sparked a mass evacuation of thousands of residents in low-lying Mackay – in the firing line for a devastating tidal surge – who were urged to leave their homes on Monday afternoon.

Premier Annastacia Palaszczuk said authorities were “very concerned” about the potential surge in Mackay, which forecasters warn could be as high as 2.5 metres above the highest astronomical tide.

“This is probably the largest evacuation we’ve ever had to do,” she told Ten News.

“This is going to be a monster of a cyclone.”

Police Commissioner Ian Stewart said there were about 25,000 residents in the affected zones but it was possible many had already left.

“We are asking people that can move out of those low-lying areas that are depicted on the local maps on the council website to move now,” he said.

“Don’t wait until tomorrow because you will not be able to move probably past midnight tonight.”

The tide is also expected to peak at 3.2 metres in Bowen, with fears of a storm surge of three to four metres above that level.

The Bureau of Meteorology has warned it will be the most severe storm to hit the state since 2011’s Cyclone Yasi.

Shortly after 5pm on Monday, Cyclone Debbie was about 205 kilometres north northeast of Mackay moving southwest at 6km/hr as a Category 3 storm.

The latest tracking map issued by the bureau predicts Debbie will weaken to a Category 2 system about 5pm on Tuesday, after it passes an area near Collinsville and Bowen.

The cyclone is expected to continue weakening as it moves inland and is forecast to be a Category 1 by 5am on Wednesday.

Townsville airport has been closed, as have more than 100 north Queensland schools.

Local farmers are also bracing for potential devastation to crops, with the Bowen area accounting for more than 90 per cent of Australian tomatoes and 95 per cent of capsicums for consumption in September and October.

Ms Palaszczuk said north Queenslanders needed to prepare for the “worst-case scenario” and pleaded with residents not to ignore police warnings to get out.

Prime Minister Malcolm Turnbull echoed the premier’s appeal and said those who had received an official evacuation order must leave immediately.

Defence force personnel were ready to respond to the looming crisis, he said.

More than 800 Queensland energy workers are also at the ready, with power issues likely.

Police say the cyclone has already contributed to one death after a 31-year-old female tourist died in a car crash near Proserpine.

Deputy Commissioner Steve Gollschewski said people had largely complied with evacuation orders in Ayr, the Whitsundays and in Bowen but some had decided to stay.

“You can shelter from wind in your house. You cannot shelter from a storm surge,” he warned.

In Townsville, wind speeds are expected to match those recorded when the region’s last severe cyclone, Yasi, struck six years ago.

An extra 25 paramedics have been deployed to the danger zone and about 600 hospital beds have been cleared.