Although i have yet to use them this site http://www.vroomvroomvroom.com.au/locations/melbourne/ seemed to offer slightly cheaper hire deals than going direct to the named companies website. I only looked at a few offers. Seems to offer no fee cancellation as well so if you find a better deal you can change your mind. Also when i booked in USA (very different i know) i managed to find some very attractive Hertz codes for some serious discounts. Invest lots of time (if you have it) trawling some discount code sites for good savings.