CLX,
Qantas will put the Q400 in anywhere they want full service Qantas brand available without the cost of a 734 or 738. They do it already on the BN-WLM sector, and despite Jet* offering $49 fares BN to WLM they still get good load factors on the Dash, at double or more ticket price!
Many big corporations have travel contracts with Qantas, their employees want a red tail and they want to re-connect to another flight without having to get their bags and stand in the Jet* check-in line again. Mining companies are a classic, they want to wait out the connection in the Q club lounge, not in the check-in line.
As for sector times, the difference between a jet and a q400 on the HBA-ML run would be measured in minutes, particularly if the q400 gets a quicker STAR into ML.