Mites are one of the most species-rich groups of arachnids, but are very poorly known. There are likely to be well in excess of 20,000 species in Australia, only around 10% of which have been named.
Mites of the genus Megisthanus are predatory and associated with semi-colonial beetles that live in rotting logs. The mites don't prey on the beetles, but on other small arthropods that live in the decaying wood. They catch a ride between logs on the beetles. Megisthanus mites are remarkably large, growing to 5 mm long.
There's still a long way to go to document the mites of Australia.