Well, it looks like uraloser and coolooa should take this outside.
In the US dredging of coastal waterways that haven't been dredged in many years is a substantial undertaking, mostly from the permitting and dredged material disposal side of things. I am not familiar with Austraila's laws concerning disposal of dredged material, but I'm sure your local government, especially whomever oversees environmental issues, would point you in the right direction - not to mention probably supply you with the right documents, etc.
If I take uraloser's response in a different vein, I bet that the beaurocratic hoops are such that doing any sort of maintenance dredging in Australia is simply impossible - hence the task is futile; you'd be better off trying to push a round rock up a sloping hill. This, owing to possibly near-total opposition to such work. My advice: Hire an environmental engineering consultant with appropriate experience and get ready for a lot of leg work.