The storm has entered the state's most populous region after pummelling the north-east coast for two days.
Although no longer a cyclone, the storm is expected to lash the state capital Brisbane with 200mm of rain on Thursday - more than its monthly average.
Queensland Deputy Premier Jackie Trad urged businesses to shut and said most public servants would be sent home.
Every school in an almost 600km (370 miles) area stretching from Agnes Water to Coolangatta would be closed, she said.
Queensland Police Commissioner Ian Stewart said the "unprecedented late call" was made after some children were already at school.
"We can't afford to have inexperienced young kids walking home from school at a time where there could be flash-flooding," he said.
The closures hit at least 1,049 schools and 937 childcare centres, according to a government website.
Conditions were expected to be "pretty horrendous" on Thursday, said Queensland Fire and Emergency Services Commissioner Katarina Carroll.
Authorities warned that flooding could also force the suspension of public transport.
Prime Minister Malcolm Turnbull said nature was "flinging her worst" at Australians.
"It is going to take some time to recover, and that's why we are here to show our support and commitment," he said from Bowen, a damaged town in northern Queensland.
Australia's Bureau of Meteorology said parts of south-east Queensland could see up to 400mm of rain on Thursday.
"What we know is that we are anticipating the heavy falls in a short duration of time," senior forecaster Matthew Bass said.
Cyclone Debbie made landfall in north-east Queensland as a category four system on Tuesday.
Authorities said 61,000 homes remained without power, and there was major damage to buildings, sugar cane and tomato crops.
The cyclone is also likely to have damaged the Great Barrier Reef, marine experts warned.
More than 50 people were being rescued from cars trapped in floodwaters on Thursday, authorities said.
All schools in south-east Queensland have been closed amid fears of flash-flooding caused by ex-Cyclone Debbie.