The preceding information in this thread is outdated! Since 18 May 2012 Ketamine is now a border controlled drug listed in the Criminal Code Act 1995 and that means that MXE no longer gets a free pass through the analogue catch-all clause at this level of legislation.
MXE is now, at best, in a legal grey area at the Commonwealth level.
This change was slipped through parliament with a huge number of other completely unrelated acts and was not the focus of much discussion. There was a brief mention of quantities and purity which suggests that this amendment was not made to specifically target MXE, though it will probably still be used that way:
Sources:
Hansard
Criminal Code Act 1995