Wow! Henry, you played both of your sets. I do not believe that either British or Australian contract law is any different than it is in the US. Hell, the US version came out of the british version and I'm assuming the same is likely for Australia as well. For almost $5k, it may very well be in your interests to contact some of your mates in Melbourne and have an attorney there take up this case for you. You should not need to ever show up in person. That excuse about "slander" is bullshit for a couple reasons. 1.) It's not "slander" if it's true. 2.) It has absolutely no bearing on the agreement you guys entered into which you completely fulfilled on your end. Had you walked away from this earlier, yeah, you'd have no claim. But, given you completely fulfilled your obligations, he really has no defense.
Did you get a contract? If not, the e-mail correspondences of agreement between you two may be enough. Either way, for $5k, it's probably worth talking to someone down there if you haven't and seeing if it's something both realistic and affordable for you to pursue.