Officials warned downtown Brisbane could be inundated by Thursday. Brisbane’s mayor, Campbell Newman, said local officials expected about 6,500 properties to be flooded in the days to come.Elsewhere in Queensland, the situation was worse. Flash floods roared through the city of Toowoomba, bringing the death toll from the ongoing floods to 18, with scores more missing.

“Today is very significant, tomorrow is bad, and Thursday is going to be devastating for the residents and businesses affected,” Brisbane city mayor Campbell Newman was quoted as saying on the BBC. About 6500 properties in Australia’s third largest city are expected to be flooded.

The warnings come after a flash flood swept through Queensland’s Lockyer Valley on Monday, lifting houses off their foundations and smashing cars into trees.

The valley funnelled rain from a freak storm — forecasters estimated up to 150 millimetres fell in half an hour near the city of Toowoomba — into a fast-moving stream that left a path of destruction.

The raging floodwaters killed at least 10 people, including a four-year-old boy who reportedly died as rescuers tried to save him and his family.

Queensland Premier Anna Bligh said 78 others were still unaccounted for late Tuesday. Emergency crews were still searching for survivors, but Bligh said she had “very grave concerns” for at least 18 of the missing.

Thunderstorms and more driving rain hampered the search and rescue effort, although the bad weather eased during the day, and Bligh said the search would get easier on Wednesday.

Brisbane is protected by a large dam built upstream after floods devastated the downtown in 1974. But the reservoir is full, and officials had to release water, causing low-level flooding in the city, Newman said. The alternative was a much worse torrent.

The Australian Red Cross said it was setting up an evacuation centre in the Brisbane area that could accommodate up to 3,000 people.