If you're talking about preassembled one from the likes of BestBuy, CompUSA, etc, save yourself some cash and make it yourself. Look at cool running drives and this category on Newegg. The higher the capacity you're looking for, the more you'll save by doing it yourself.
If you travel with yoru laptop frequently, I strongly recommend that you get a 2.5" laptop hard drive and enclosure. With the smaller drive, you can use a bus powered enslosure that doesn't need a power brick, and the drive is much likely to survive falls that will happen while you're on the road. A notebook drive can take a fall while it's running that would kill a non-operatign desktop drive - there's that big of a difference in shock resistance.
Though newegg isn't going to do him much good since he is in Far North West Queensland, Australia
Your best bet is the sunday markets there are usualy several local and non local computer vendors at any of them
I dont know if there is one in your town specificly but there is very likly to be one within 50km
Failing that www.pacstarcomputer.com.au www.ht.com.au
will give you an idea of what prices you should expect to pay
PM me if you want me to look up some more places you could order from in sydney to get shipped up to you.