aha! a fellow aussie! i'd probably be taking mooz's advice a bit mroe to heart than mine, as i think he has lots more experience than me.
If you're looking for a reliable, fairly cheap place to hire in Cooma, rhythm snowboards is the place to go, ask for Ducky.
But, if you're set on buying your board, Sierra Snowboards has a pretty good sale on at the moment (stay away from the Burton products, i don't think they even ship to Australia anyways), or you could go for the Rossignol (i personally haven't ridden one, nor do i know anyone that has, so i can't help you there).
Personally i'd go with the 159, just because i have a park preference, and also because fresh pow is virtually non existent in thredbo and perisher :P.
But then again, i have a very limited understanding of how riding preference affects board length, all i know is park boards are shorter, backcountry/off piste boards are loner. Mooz has some pretty good advice there, or you can just wait until someone posts with some more info.
p.s. I also remember last year, the shop directly opposite IGA or whatever it is in jindabyne (nuggets crossing centre thing i think) theres was a rent 5 days thing board and boots for $70 which is pretty decent. I haven't dealt with hem though, so i dunno about the quality.
Cya cruisin down high noon