[dave295]

Check out the Abandoned Goods legislation in Qld.
It would probably cover any requirement to advertise "the goods" depending on your valuation of them, prior to claiming ownership of them.
If you then own the goods you can dispose of them as you see fit.
You could also consider placing a lien on the goods for storage after a reasonable period of time had passed (say 12 months) leaving about 9 years. The goods would not be discharged to anyone until the lien was satisfied.
Ask your local Police station or community legal advice centre for advice.
In this situation it sounds like it is not a motor vehicle, it is only goods.

What would you do if a flatmate or tenant moved out leaving a bed, wardrobe, or other personal effects etc. behind and didn't pick them up in a reasonable time frame?

Dave