I would like to make an award reservation (one-way) by using BA miles.

BNE ~ SYD (stop)
SYD ~ MEL (stop)
MEL ~ PER (destination)

I found leg by leg using the one way search function at Quantas.
Then, call to BA for stopover award.
However, I was told that this route is not eligible to stopover, rather it is side trip (I don't get what that means)
If anyone has idea or suggestion, please let me know.
I would very much appreciate it.