According to the Australian Customs Service, there is a duty free concession for things that are:
- owned and used by you overseas for 12 months or more
- imported temporarily (a security may be required by Customs and Border Protection)
Most everything else would be covered by the AU$900 limit (see the webpage for the details and some caveats). Technically, as the bit about posting a “security” suggests, expensive things might still need to be declared but not taxed.
It applies to everything that hasn't been properly taxed in Australia, so things bought abroad, purchased duty-free in Australia or things on which you claimed a tax refund.
In practice, most people don't bother (arguably a used laptop and smartphones might not be worth that much) and many get away with smuggling new goods over the limit (those would definitely need to be taxed).
That limit is actually quite high, in the EU, it's €430 so about $600. In particular, once you do live in a country (citizen or PR or not, that's usually not relevant as such), you are definitely not supposed to buy a high-end laptop abroad and bring it back tax-free. But travelling back-and-forth with a laptop you are mainly using abroad is allowed under the aforementioned concession. And returning to Australia with an Australia-purchased laptop is obviously allowed too.
Note that while I don't know the exact rules in Australia but there are also typically allowances for personal effects when you move into a country. So if you bought something elsewhere before becoming a resident, bring it with you, and then travel in and out of the country with it, it should also be OK.
But if you bought an expensive gadget after effectively residing in the country, you would need to declare it and pay all applicable duties and taxes, after which it should be treated like something you bought in Australia.
In both cases, it might still look as if the goods weren't purchased in Australia (based on the serial number or some other characteristics) so it's best to retain all relevant documentation to be able to prove when you moved and that you completed all required formalities. (On a related note, some customs administrations recommend not to travel with a lot of jewellery if you don't have a proper appraisal because it might simply be impossible for you to prove that you really bought them in the country.)