Hi jeongse1,

It may seem obvious, but I assume you want to fly BNE-SYD-MEL-PER one-way because, basically, those are the places you want to visit.

Unfortunately, a BA one-way award for 25k (economy), is not simply a routing along the points you want to visit. It's a trip defined initially by a specific starting point and destination, then constructed along a "simple" route.

So for 25k you can probably go BNE-MEL via SYD, BNE-PER via SYD (*or* maybe MEL), or SYD-PER via MEL. But if you want the whole trip it won't work as that's not a simple enough route from BNE-PER; it would have to be one trip BNE-SYD-MEL and another MEL-PER, or something like that.

I would guess you're best offer trying to book SYD-MEL-PER and then buying a ticket BNE-SYD.