DANGER ZONE: These bull sharks were recently snagged by Chris Molloy while fishing at Kookaburra Park in Karana Downs.

DANGER ZONE: These bull sharks were recently snagged by Chris Molloy while fishing at Kookaburra Park in Karana Downs. Contributed

DUR-dum, dur-dum.

You might want to reconsider taking a dip in the Brisbane River once you see what's lurking beneath the water.

Bull sharks seem to be a common find in the Ipswich region waterways with more instances of the predatory fish being hauled out by local fishermen.

During a recent fishing session at Kookaburra Park in Karana Downs, 26-year-old Chris Molloy and his two friends snagged three bull sharks - each more than a metre in length.

The sharks were apparently schooling together and pushing mullet and bony bream up the river. Mr Molloy caught a pup about 1.1m long, while his mates Colin Freeman and Liam Singh caught the other two - which measured at 1m and 1.6m.

But they were small-fry compared to "the one that got away".

Mr Freeman lost a 200lb metal trace trying to haul in one of the sharks which the three men believed to be at least 2m in length.

Mr Molloy's mum, Jenni, said she was shocked when she saw what her son and his friends had caught.

"I was surprised, not only by the size of them, but by the amount of them in our waterways," she said.

"I've heard reports the sharks have also been sighted between Mt Crosby and Colleges Crossing."

Ms Molloy said she wondered how many people were even aware that the sharks were in the river.

"People take boats out and go water skiing and tubing and swimming in it," she said.

"Some small children wouldn't stand a chance if one of those sharks attacked them."

Australian shark scientist Dr Jonathan Werry has worked with tracking bull sharks since 2005 and completed his PhD on bull shark movements in urban and natural habitats.

Dr Werry said juvenile bull sharks used very defined low salinity areas of the upper reaches of river systems.

"The low salinity area moves up and down a river system influenced by tide," he said.

"If a fisher happens to fish within the right salinity range, there is the potential to capture a number of small juveniles in one sitting."

Dr Werry said bull sharks were found in almost every river system in Queensland and there was always a risk of swimmers encountering them, especially if the water was murky.

"During the day, they tend to remain near the river bottom and during the night they are more active near the surface," he said.

"So people should avoid swimming in the river around dusk and dawn when bull shark activity increases.

"In most cases, however, small bull sharks tend to avoid humans."