David Pomery, chief weeds officer with the Illawarra District Weeds Authority (IDWA), has been battling an infestation of water hyacinth at Coomaditchie Lagoon since February.

"It was likely introduced to the lagoon accidentally or intentionally by someone dumping plants there," he said.

"It can double in size every week under the right conditions.

"Once it takes over it dominates the ecosystem, outcompeting native plants and any aquatic life underneath."

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Unfortunately for the IDWA the sensitive nature of the site coupled with heavy rains have limited access over past weeks, and the water hyacinth has exploded across the northern side of the lagoon.

Mr Pomery said they were hopeful of getting it under control - they just need enough dry weather to get to work.

In the meantime he urged local gardeners to be mindful of how they dispose of backyard plants.

"Water hyacinth is a state priority weed under the biosecurity act, because it is so invasive," he said.

"It can devastate the natural environment.

"If you see something spreading in your own backyard, be conscious it will do the same thing if you dump it elsewhere.

"If you have something you are unsure how to dispose of, contact us for advice.

"We're not in the business of handing out fines, but would be happy to take the weed out of your garden and recommend friendlier replacements."

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