At least 10 people were killed and 78 are still unaccounted for.
Meanwhile, the Brisbane River overflowed its banks on Tuesday and officials warned that dozens of low-lying neighborhoods and parts of downtown could be inundated in coming days.
Brisbane Mayor Campbell Newman said authorities were preparing for flooding affecting about 15,000 people in 80 suburbs.
But nothing downstream was expected to be as fierce as the flash flood that struck Toowoomba on Monday. It was sparked by a freak storm - up to 6 inches fell in half an hour.
The raging water was strong enough to rip houses off their foundations.
Authorities said the price of rebuilding homes, businesses and infrastructure, coupled with economic losses, could be as high as $5 billion.
The Associated Press contributed to this report.