The current hypothesis is that the two-headed, four-eyed critters have been horribly misshapen due to....macadamia nuts? According to Discover.com (the article is worth a read just for the phrase, "Lando sent a report...", if you're a Star Wars nut like me), "noxious fumes" from a macadamia nut farm may be contaminating the water in the Noosa River.
Ah, but if you read further, it's our old friends, the pesticides, not the gentle nut, that appear to be the culprit. But with the geniuses in charge of the government saying helpful things like, "Fish don't have two heads, they generally have one," it's truly amazing that they have gotten this far in the investigation.
In more positive two-headed fish news, I see that they have been used to study the genetics of conjoined twins, which is cool. And, a double-headed lure may catch you more fish. If you're into that sort of thing.
[I must note that, disappointingly, Discover got the name wrong: it's Landos, not Lando. And he doesn't even slightly resemble everyone's second favorite scoundrel. Boo!]