Heavy rains have filled Wyaralong beyond capacity, but the floodwaters have also filled the dam with something of a far more problematic nature.

Water Hyacinth, one of the world's worst aquatic weeds, is blooming in the warm, nutrient rich floodwaters.

Boonah’s Trevor Elliot, who regularly fishes Wyaralong, said the outbreak emerged after recent floods and was getting “worse and worse”.

“Lots of it is almost unfishable from the shore,” he said.