Three family members in one car drowned - a 74-year-old man, 39-year-old woman and a boy, 5.
A 21-year-old woman and her 16-year-old brother escaped from another car but 12 hours later, the body of their father was recovered 100m downstream.
In nearby Burpengary, firefighters rescued a woman who was clinging to a tree after she and her 75-year-old husband drove into floodwaters. His body was found more than six hours later.
An investigation was under way into each incident, Inspector Lee Jeffries said.
"I haven't seen anything as significant and as tragic as this."
Moreton Bay mayor Allan Sutherland said the SES was stretched to the limit and more than 200 roads had been closed.
For five hours at the height of the system, emergency services received more than 4000 triple-zero calls - 13 a minute.
There were nearly 246 swift-water rescue incidents and 58 rescues were performed. But it was not fair that emergency service personnel were being forced to risk their own safety to rescue residents who ignored warnings, Emergency Services Minister Jo-Ann Miller said.
- AAP