hi thanks for the query its nice to have com channels flowing.
i'm not aussie ATC but i know in our country we have to have then miles sep behind an a/c doing rnp approach something about ATC not being able to manipulate traffic's speed on the approach.so if you fly a/c that is slower than the rest of the traffic that would have to be increased even more for the faster one behind.so then i can imagine they don't want to hold up faster boys on the downwind until you get closer to threshold.
this is my thumb suck explanation,hope it helps...like i said i don't know much about aussie rules ATC and i don't work at a station that has published rnp approaches.
