If the IREX exam, you should already know enough to do the questions not related to legislation but you will need to know the Oz air law side, procedures etc which are a major part of the exam and that won't be the same as US. Maybe just buy a book (eg Bob Tait's Instrument Rating, or similar) and study it - you don't have to do a course, you can self study. There is an example of typical IREX questions on the CASA web site
https://www.casa.gov.au/standard-pag...tion-questions that will give you an idea of what is in it. Where most people come undone in the IREX is not their lack of knowledge but not carefully reading the questions to see what bit of knowledge they are actually testing, the questions often seem to be worded to deliberately lead you astray so you think you know what they are asking about but on careful reading and teasing it apart you realise it is something quite different.