Did you know that VicRoads recommends you don't? If your car has a lap-sash seatbelt, the one that goes from shoulder to opposite hip, with a lap belt as well, then they recommend not using any sort of additional child safety harness.

Are child safety harnesses recommended?
No. A child safety harness is difficult to use and fit correctly and is commonly misused. Incorrect use of the child safety harness can cause the seatbelt to ride up into the child's stomach area, which may cause injury or death to a child in a crash.

Do not use a child safety harness where a lap-sash seatbelt is available. Research recommends using a child safety harness only in situations where it is not possible to replace your lap-only seatbelt with a lap-sash seatbelt.

If you are not able to replace your lap-only seatbelt with a lap-sash seatbelt you will need to use a child safety harness with a booster seat. In this case, it is recommended to use a booster seat with an anti-submarining feature to hold the lap part of the seatbelt down low on the child’s hips.
From VicRoads - Child Safety Harnesses

We've currently got them on DD's Boston Hipod. Think I might be removing them...