The Water Corporation says people illegally accessing the Harvey Dam crest are risking their lives, with the dam tipped to overflow at any moment.
A photo has emerged on social media of people in a canoe crossing the 40m dam wall, which is more than 25m high on the spillway side.
The dam has a capacity of 56 gigalitres and is currently sitting just below that mark.
An exclusion zone of 150m from the spillway crest is in place.
The Corporation's Phil Kneebone says it is incredibly dangerous for anyone to be on the dam crest.
"It will be incredibly slippery and if someone was on that spillway and slipped, they could easily go down the five metres and then the next step after that is a 20m drop onto rocks down below," he said.
"That's about half the height of the Bunbury tower and we're incredibly concerned about it obviously and we're not taking it lightly and we're just warning anyone to stay away from the spillway crest and also not go out there in canoes or boats in any description."