From my experience booking flights from YYZ -SYD/MEL return:

1. Have booked it as two one ways when I didn't want stops in either direction
Was told by agents that after you book two one ways you can not then combine them together in order to get a stop. They must remain as two one ways.

2. Recently have booked for Feb/March 2016 -- Booked a return where I wanted stops both ways. First booked "place holder" return MEL - YYZ (any date that had space) coupled with YYZ-SIN-MEL (stop in SIN) when it opened. When MEL-YYZ space opened up (six weeks later) I called in and had my "place holder return" changed to the date and flights I wanted which include a stop in Paris. Cost was $90 ($101 with tax) plus the extra YQ/fees if routing includes YQ airlines or refund if your change eliminates some YQ airlines.