There is a lot that lurks in the water of the Brisbane River, but among the fish and mangroves people are finding bullets, trunks of pornography and lost bikes.

Magnet fishing is where environmentalism meets treasure hunting, and it's becoming a popular method of cleaning up Brisbane's waterways.

The activity involves using a rope with a strong magnet attached to pull items to the surface, but sometimes it isn't that simple.

The magnetic bounty has included shopping trolleys, car wheels and fishing tackle. ( Supplied: Adam Gee )

"I've had items where it's been a two or three-man job because it's so heavy," Adam Gee said.

"The thrill of the find is really the big part of it."

Mr Gee took up magnet fishing after seeing his father search for meteorites.

Loading

"He was meteorite hunting with magnets and that then turned me to magnet fishing, which I saw on YouTube," he said.

"I really loved it and I'm always asking, 'What will I find next?' Could it be a bike, or could it be a trolley?"

The right fishing tackle

Mr Gee's equipment of choice is a 20-metre nylon rope with a 115-kilogram-pull-force magnet.

Adam Gee uses a magnet attached to a nylon rope to drag the river bottom. ( ABC Radio Brisbane: Adam Gee )

Bites happen when the magnet touches other magnetic material and pulls at the rope.

"It really comes down to luck as you can't see the bottom of the water and you really don't know what's going to be there," he said.

"You can only pick up other material that's magnetic.

"I've pulled up a lot of street signage, trolleys ... there are always trolleys."

A trunk found in the river contained adult magazines. ( Supplied: Adam Gee )

Shot gun bullets, lost car keys, trunks of pornographic magazines and car tyres have all been discovered by Mr Gee's fellow magnet fishers.

He said the best thing he had found was a handmade shank, yet the most recovered item was fishing tackle.

"I do pull up a large amount of fishing gear — it's unreal — from lures to hooks.

"When we pull it up it helps the environment and cleans up the waterways, and in high-fishing areas you can get fishing line up by the bucket load."

Choosing a fishing spot

Mr Gee said choosing the right spot usually came down to the amount of foot traffic or boat traffic in a particular area.

Ammunition retrieved from the bottom of the Brisbane River. ( Supplied: Andrew Gee )

"I use Google Maps and recommendations from other magnet fishermen," he said.

"A lot of it comes from Facebook too, as we have a group where people post what they've found and where they've found it, and that pushes me to find different areas."