Even if they don't do anything, those little plastic scarers are a good placebo. I don't like talking about hitting roos ( Murphy's Law and all that ) but I have hit a few locally just driving to work in the dark, no scarers.
But I have done a lot of Commodore outback touring during the past 10 years with no problems (yet). Just by not driving at night, only good daylight hours, cuts your chances of an incident. ( If you see a wedgetail eagle feeding on road kill, you give way to them as they won't just fly away.)
Good onya for wanting to spend the $$$ in Straya, but I would recommend not driving Broome > Darwin > Cairns during summer months. Most of the tourist attraction places are closed, and you could miss out on something that you really wanted to see.