It's clear as mud.
I've cut this out of the Vic Roads RW guide:
"The outlet must be located so as to avoid direct entry of gases into the passenger compartment and must not extend beyond the plan of the vehicle."
http://www.vicroads.vic.gov.au/NR/rdonlyres/AA944BA2-1210-460F-94BF-069159D224F8/0/VSI26web.pdf - page 6
There is nothing in any literature that I can source to say it must come out the back or a particular side. There are lots of people who say it must do this and do that or it's (?) illegal, but they don't quote any source or reference.
I have found restrictions on vertical pipes and catalyst and silencer and emissions but nothing else on design, other than it can't run through the cabin, and from another Vic Roads mod sheet (link in earlier post) that regulates protrusions.
I recall reading many (and I mean many) years ago documents regarding exhaust exit angles (up, down, left & right), exit distances/clearances, but I can't find any such documents that are now current.
It would appear to me that as long as it is secure, clears the road by at least 100mm, doesn't leak (water drain holes are NOT leaks), meets the relevant noise emissions (98dB for most of us) and isn't likely to cut or burn anyone - it would meet all the requirements. (emissions are a seperate issue )
Hope this helps.
MaygZ