Is the overall SC earn correct for the price you paid?
This is rather interesting, as would be whether a return itinerary (with exactly the same sectors both ways) would be "symmetrically" credited. That is, if we take your example, if it were a return itinerary on the same legs and exactly double the cost of what you paid, would you receive the same SCs in reverse?
In some way, if the overall SC given is correct for the price, not many people would care too much about how the earn gets split up per sector. It probably only matters if something irregular happens while the booking is in progress. For example, if someone books BNE-PER return, flies the BNE-PER successfully first, but then on the return gets rebooked to PER-SYD-BNE (for whatever reason), what's the earn there? (One answer is that the "fare paid" used for calculating SC doesn't change - it's based on what you originally paid, no matter how much your ticket then subsequently changes)