If you do a weblookup on importing a vehicle into Australia you should be able to get all the info you need from Australian Customs, Australian Quarantine Service (AQIS) and Dept. of Infrastructure (lookup VSB10). There are strict import requirements, make sure you comply - eg carnet de passage.
In a nutshell - AQIS can demand that a used vehicle is cleaned "as new". In reality if all mud is removed from the underside, and the interior is cleaned to a high standard you'll get it through. No water in tanks or washer bottle, put a bug strip inside the vehicle, no uncooked food (tins OK, rice, flour, etc not OK). You'll find a vehicle cleaning checklist on the AQIS website.
Grease caked in mud will get you a knockback and a clean at your cost.
Look at all of the AQIS "not allowed" items and just check off that the vehicle does not have them - plywood is OK, ordinary timber is likely to get the works at your cost.
Biggest thing is to be present at the inspection, if you allow an agent to be there in your place I can almost guarantee that your vehicle will need a clean and the costs will double. AQIS and the agent may have a too cuddly relationship.
With vehicles always have the AQIS inspection done at the wharf, if the vehicle is taken away and is found to need a clean it will be returned to the wharf or to an approved cleaning station. Cleaning stations are few and far between for AQIS and can charge a small fortune.