I'm pretty sure that you have to present a receipt to show the value of the goods on arrival, so they can work out duties and GST on the spot. I did once bring a car part from Australia to New Zealand for my father, and New Zealand Customs asked me for a receipt on arrival. I didn't have one, but luckily they let me off with a warning to always bring one in future!
I haven't imported anything while traveling into Australia, but when my partner imported some electronics recently from the US by freight for his business, duties seemed fairly small - it was GST that was the big one! So, if you aren't already registered for GST, you should do that soon so you can claim it back.