Hi, i am at Wooli NSW and wallked today into Yuraygir NP to Wilson's Headland. I saw a bird that i could have sworn was a brown Kookaburra. I haven't been able to find any other bird like it in the ID apps, but i did find a reference to and old post on BIBY which is no longer live. So it might be a colour variation Kookaburra eh? I only had my phone with me and it wasn't up to getting a pic in the conditions.
There is an article on melanism in kookaburras on the Qld Museum website here that might throw light on this.