Australia Post have a postage calculator on their website. This is identical to the one used by post Office staff, so if you enter the details correctly, you will be quoted the exact same price as if you took the parcel to the post office and had them do the calculation. I use it regularly for local and international postage calculations and get the correct price virtually every time.

As Dave has mentioned 3 kg and 5 kg. satchels work out much cheaper than packing items in a box. For those who dont know ( and there are quite a few ebayers who dont use them ). The satchels are a plastic bag with pre paid postage that you buy from the post office or through ebay. They have a sealable strip at the top and places for address and air freight declaration signiture. They also have a tracking number sticker that you can peel off and keep. You just put your packed item in, seal and mail over the counter at AP office or put them in the mail box. If you put them over the AP counter you can see the staff scan them in and ask for a scanning reciept.