Just checked you PC vendor site - you got some nice selection down under, we'll pick sth, worry not.
S12II-430 / 520 Bronze non-modular are 150 x 140 x 86. I've the 430 and it sure is small. A capable and v quiet unit. My main system (moderately OCed CPU & GPU) draws about 280W at the wall during Stalker. By this time the stock VGA cooler is so loud all other noise is not a problem. Point is - if your system draws a similar amount of power or more, your CPU and GPU fans will most likely drown out the PSU. And in idle the S12 is drowned out by my HDD (coolers & fans being v v quiet in idle). Sure, it's not super quiet and won't stop its fan like the newer SS do, but it's a budget unit, duh.
Check other Seasonics for size:http://www.seasonic.com/product/pc_retail.jsp
Also, Antec EarthWatts EA500 is 150 x 140 x 86 - it's only 80Plus, but perhaps you can live with that. Also, it has a back mounted 80 mm fan, which I assume may get slightly louder under load - but gain, once you really load your PC, other fans will chime in and the PSU may not be a noise factor anymore.
The NAXN 500W is a small unit, but I haven't read any reviews on it. Still, Enermaxes are generally v quiet, so mebbe worth a try?
Personally, I'd go with one of the S12 - I've had SS PSU since I discovered the site in 2005. I'm sold to SS for life