I can appreciate AU being so particular about the cleanliness of equipment entering their country.  The US has requirements regarding cleaning combines when custom harvesters complete a weedy field but hardly a thorough cleaning.  Cleaning consists of a blast of air from an air hose that does little good.  On our farm in NW KS we see dozens of weeds we didn't see even 30 years ago that have been brought in by migrating birds or wind blown seeds. 
Some like Musk thistle have been imported for their "beauty"  but have become a nemesis to no end.  The US spends Billions of dollars each year trying to control them.  I personally spend about 200 hours per year on a 4 wheeler just controlling them in the pastures.  A Musk thistle seed can lie dormant for as long as 60 years and then sprout.  Each plant provides millions of seeds for the next crop.  We now have weeds that are resistant to normal chemicals and that causes much extra expense. 
It wouldn't take much for some sort of noxious seed to be wedged in a small crack or gap on a tractor only to dislodge some years later to begin its life cycle all over again.
Back in the late 1960's we even had a custom harvester leave a coral snake in our yard that must have made the ride from Texas as they are not native to this region.
 

it a big thing even crossing state borders with farm machinery. we have the strictest quarentine laws in the world hence why our produce is welcome just about everwhere.

the inspectors will make you reclean any machinery to their satisfaction and all at your cost.