The 47-year-old woman was near Mount Usher Falls in Bouldercombe Gorge Resources Reserve on Sunday afternoon when a heavy thunderstorm dumped torrential rain on the area.
Police say the noise from the approaching "wall of water" sent people scurrying to the bankside for safety.
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As the water tore past it swept away the woman, her 17-year-old son and her sister, who had been on a creek crossing, part of a walking track to Mount Usher Falls.
They were visiting the area with other relatives and friends.
"The woman, her son and her sister were washed downstream in the torrent and her son managed to grab hold of her in an attempt to pull her to safety," a police statement said.
"The power of the water was too intense and tragically she was wrenched from his grip.
"She was located deceased a considerable distance down stream."
Boy in wheelchair drowns
Others in the area were also swept away and located later, while some managed to scramble onto a nearby island to await rescue.
Reports of the total number rescued vary. Police said 18 people were located uninjured, while a Department of Community Safety spokesman said the total number involved, including the deceased mother, was 21.
A group of four teenagers were among those rescued from the island.
Meanwhile, a 14-year-old boy drowned when the car in which he was travelling tried to cross Black Duck Creek at Junction View, south of Toowoomba, on Saturday night.
The boy and two other people managed to get out of a car after it was swept off the roadway, but the boy was washed away in the floodwaters.
Channel Nine reported that the teenager was confined to a wheelchair and his father had valiantly tried to rescue him.
Torrential rain in southeast Queensland on Sunday morning caused havoc for emergency services and water police who rescued 20 people and retrieved boats that came loose from their moorings and floated down a Gold Coast river.
Clagiraba in the Gold Coast hinterland received 415mm of rain in the 24 hours to 9am (AEST) on Sunday. Nearby Canungra received 366mm and Mt Tamborine 372mm.
