Assuming the goal is to get to Perth in one day, flying at power/altitude/weight which produces the highest TAS will get you there sooner, but it may put stress on the engine.
Don't know which aircrafts you are flying, but some POH's have "Power-Density Altitude-TAS", and "Power-Fuel Flow" charts. Bob Taits CPL Performance book provide some answers how to resolve these problems.
Also allow time for refuelling, rest, etc..
Many people do it in two stages staying overnight in Forrest.
The best way is to enter the route details into EFB, then analyse the results. Below is the extreme case scenario without stops, direct lines. Perth EOD today is 11:48UTC, subtract 10mins for landing time, and also departing early morning at BOD will give you another ~5 hours.