Hi Steve,
Just wondered how you got on with this? I'm in the same situation as you and have just received my email to say that I can sit the test on 25th Jan '19 - but I'm in the UK now. I actually can't afford to fly back so am hoping I can sit the test in the UK - haven't called immigration yet just in case it affects my application.
I know that you are free to leave Aus after you've sat the test and then just fly back for the ceremony but I couldn't wait in Aus to have the test, due to multiple reasons, and had to leave Aus before my test invite came through (that in itself was a 13 month wait).
Keen to hear what route you've gone down and if you've flown to Aus for the test with no issues.