Optus is now offering trade-ins for some older phones when you buy a new one on contract. That might sound tempting, but we'd suggest using a different strategy to maximise your return.
The deal sounds simple enough. Take your old phone -- which must be able to power on, and which you should wipe first -- to an Optus store, along with some ID. You'll receive an on-the-spot offer, which you can put towards paying for a new phone on a contract plan. Not all models are covered, but all iPhones and most recent Android models are on there. (You can see the full list on Optus' site.)
There are two big problems with this arrangement. Firstly, you can't research Optus' offer in advance. Optus will allow you to trade in within seven days after buying a new phone (so you could potentially sign up and then take advantage of the refund after the fact), but the price is only valid at the time the offer is made in store. If you come in 24 hours later (after checking the alternatives), it might be different.
The second issue is that this only works if you sign up for a phone contract. We're not fans of contract phone deals, and we'd argue that you can almost always do better with a month-to-month deal.
If you have an old phone in working order and you want to make some money from it when you upgrade, we'd recommend checking the various buyback sites and going with the one that offers the best deal. These are the active ones we currently know about:
It's worth the extra effort to maximise your return and avoid being locked into a contract.