Queensland water police have recovered a five-metre inflatable yellow duck found floating down the Brisbane River.

Police tried to tow it back to West End for this afternoon's cancer research fundraiser, The Great Brisbane Duck Race.

Vandals had cut free and slashed the duck before letting it loose.

Organiser Simone Plunkett says she is not sure if the duck is in any condition to be used as a finishing post.

"We think someone has had a little bit of fun with him overnight... and the water police found him heading out to sea earlier this morning," she said.

Later, Ms Plunkett said the big inflatable duck did not make it.

"We thought we'd actually been pretty lucky because the duck has been out on the river for six weeks and we were surprised it hadn't got a hole in it or deflated or someone might have shot a pellet it at," she said.

"But it helped serve its purpose so the race will still go on."