Well I just booked it thru the qantas book to start with, say 4 months before travel, about AUS$100 per day.
Originally ordered drop-top but ended up with hard-top which looks heaps better & it rained solid for 4 days anyway!!
Yeah push for the pony, they have heaps, I even rang Hertz at SFO from Oz to be sure I got one!!
Good Luck
Cheers
Dreemer