Well, a retailer that sells PCs, who provides a return to base warranty, have replaced my PSU with something with lower rating (750W to 460W) without informing me. They've told me this when I've asked why they've done it without informing me.
The Australia Power Supply are out of control from the importers. As a reseller, we can only tell the performance by price. So we give you a more expansive power supply to reply your cheaper one before.
They've tried to pull similar move when the GPU of mine failed, they tried to replace HD7850 to R9 265, which as far as I know worse than what I currently have.
So is this kind of behaviour a normal behaviour in Australia? What should I do in this kind of case (other than showing my utter disgust)?
That sounds like a load of crock. And, if its a major failure, you have the right to demand a full refund. That power supply would constitute a major change in the value of the machine. Who cares what it costs? If they don't have the right part to fix the computer they need to supply a new identical computer, or refund you!
https://www.accc.gov.au/consumers/consumer-rights-guarantees
They have decided that they cannot repair the power supply, and are replacing it instead. From the link above:
"Replaced products must be of an identical type to the product originally supplied."