Just had a call from a client with this question, and I actually wasn’t sure how to do this myself. After a bit of research … ta da … here is how you do it.
First of all, you must be using MYOB Premier as this is the only version that allows foreign currency. Ensure you have the multi currency preference selected in Preferences.
After this go to Lists | Currencies and if the currency already exists that you want to set as the default, delete it (or rename it to something like XYZ). Then go into the AUD currency and change this to the new currency (see how it says Local on the top right hand corner?).
Good luck, hope this helps!