You’re right Dannyboi, 'Whistling' can be used for both species (unfortunatelly), which is why I also listed the species I was referring to.
I'll also concede that the two species we’re discussing are extremely closely related, and I was a little gung-ho with the declaration that it was L.verreauxi... without listening to the males call (and even then), they can be virtually impossible to differentiate, even in the hand. It’s just that L.verreauxi do tend to be more green, more often. And no, L.verreauxi don’t always have black rather than brown, they just have the tendency to have darker markings.
There is no way to determine the absolute I.D. of this individual, and I have no doubt that you know your frogs. My primary objective with these two posts, is to discourage the release of the frog, and to hopefully point the frog in the right direction.
So the answer is that it’s either a Brown Tree Frog (L.ewingi), or a Whistling Tree Frog (L.verreauxi), and regardless of what it is, shouldn’t be released.