Just a bit more on this, but from the flood perspective have a glance at these two stories. We get fairly major floods up in the north on average probably every 5 to ten years.
Floods threaten Great Barrier Reef, La Nia to blame - CNN.com
Below is a picture I found online showing the flood plume from the Burnett river, near Abbot pt, which I have marked for clarity.
Like I said, i feel its a bit of a tree hugger beat up, but I am by no means an expert on flood damage or dredging spoil damage, but I do know that there is 25 NM between Abbot pt and the reef and that flood plume is almost there in the picture.
Further south at Great Keppell Island there is reef surrounding it and its only a few miles from the coast.
Supposedly there was 100% coral death from the sediment dumped on it from the last flood so say some news reports.