I dealt with this last year, my UK ATPL with English Proficiency was not good enough for CASA and I had to do a proficiency check. Your two options are to find an approved flight testing officer at a flying school, I would think where ever you are going to be based for your flying out in oz will require you to do a check ride, you may be able to know off two birds with one stone. You can also book a proficiency check with a group call ASL (Aviation Services Ltd) who conduct all aviation written exams. This is what I did, it cost me $100 for the experience. It was the most ludicrous things I have even been asked to do in any of my flying career (and that is saying something). You go to the testing centre and then sit in a room and talk down a phone line at a computer who asks you to record responses to questions. You then wait 7 days for the result to be determined before they turn around and say 'hey, you can speak english!!'.
Of course your other option is to pop into your flying school in the UK and ask the resident 'checker' to fill out the form for level six english. Pay the UK CAA 25 quid and bobs your uncle. The only reason the UK licence has no 'level' attached to it is because the gave everyone with a current licence ' grandfather rights' of english proficiency with everyone to be assessed at somepoint in the future when they complete their next checkride.