Upvote:14
If you are 18 or older, Australia allows you to bring AUD900 of "general goods", which includes electronics, without paying duty.
If you exceed AUD900 in goods, you are liable to pay duty and tax on all of the goods you're bringing in, not just the excess, so make sure not to exceed it (so for example, if you buy your PC parts for AUD850, don't also buy some jewellery for AUD100, since you'll then be taxed on all AUD950 of your goods).
Under the duty free limit, you do not need to declare your goods, but you should make sure to hold on to your receipts, in case you are inspected at customs.