An unprecedented mass evacuation is being carried out at the last minute in the Queensland town of Mackay as "severe" Cyclone Debbie gains intensity and threatens to cause widespread destruction in conjunction with a high tide.

QLD Police commissioner Ian Stewart said authorities were urging about 25,000 residents in "low-lying areas" of Mackay to evacuate their homes immediately, according to news.com.au.

"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 ... assist family members and the vulnerable to move."

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Stewart this afternoon told media there were grave concerns for the coastal town because it is a "low-lying" area that is likely to be affected by a 0.8-2.5m storm surge, caused by a dangerous combination of a category 4 cyclone and high tide.

Mackay has a total population of more than 100,000 people. Evacuations are now underway with authorities instructing remaining residents to check the council website to find out if their homes fall into an evacuation zone.

"This is a very big call on our community," Stewart said.

"We've certainly moved whole town before but nothing of this magnitude.

"This is right across Mackay because of the unique landscape and low-lying areas."

Stewart said police were asking people "in safer areas to open their houses and hearts to their friends".

"We don't want to see any tragedies as a result of this storm," he said.

QLD Premier Annastacia said it was "very clear the time to move is now".

"This is a severe weather system," she said.

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