Like Quandary we also brought our bikes beck to Brisbane from New Zealand after 4 weeks of every dirt road we could find around the South Island. Had so much fun we are going back again this year
They were absolutely filthy when we started cleaning them, and even after that there was a lot of road dust in places that we just could not get to or more accurately could not be bothered getting to, like under the battery etc etc etc. But they can't even look there without you being there to open up the bike and I have never heard of anyone who has a bike that "looks" like it has had some effort put in to clean it appropriately being asked to dismantle their bike.
We spent half a day cleaning the bikes before shipping from Christchurch, pressure washed them first and then cleaned them with car detergent then a cap full Keosene in a bucket of water made em shine so they looked shmikko.
The most important thing is that there are no bug guts or dirt in places they look for it such as under the mudguards and in the tyre tread, think like an inspector and go over the bike looking in every nook and cranny you can easily look into.
Put in a bit of effort and it will be fine, IMO there is a lot of scare mongering about Oz quarantine, the bike MUST be clean as a whistle, but it does not have to be showroom shiny. They are worried about disease pathogens and gremlins that sneak in not how spiffy the bike looks.