John:
Of course its true. Barra are spawned in the estuaries during the start of the wet season. They then disperse upstream during the wet-season flooding. Anywhere they can swim to, a saltwater croc can swim to also. The crocs can also walk overland to impoundments where barra cannot go. No barra doesn't necessarily mean no crocs!

I use the tell-tale signs of tarpon rolling as a general rule, tarpon are visible by their air-gulping behaviour, whereas barra are usually not visible from the shore.

If tarpon are rolling, there certainly could be salties there.

However, I agree with Bakes. NOTHING would convince me to stand thigh-deep in a billabong or river north of about Tennant Creek!
...unless there was a hundred-foot waterfall just downstream!