A trackable is a generic term that refers to an item (and sometimes a hitch hiker) that carries a unique tracking number and can be logged when visiting caches. Trackables usually take the form of a travel bug or a geocoin. geocaching.com has their own TravelBug(TM) system.
Can Geocaching.com trackables be placed in Geocaching Australia caches?
While possible, it is a practice that should be discouraged as the gc.com site does not have a mechanism in place to show that the item has been dropped off in a non-gc cache.
The trackable item then goes into limbo, showing it as still being held by the last cacher. People setting out to seek the item will not be able to easily tell it's actual location. Their stay in a GCA cache also will not count towards any statistics belonging to that trackable.
GCA caches do not have a facility to show what GC trackables are held within them, nor does a GCA GPX export contain a list of the GC trackables held within.
Are their alternatives that are supported by Geocaching Australia?
Yes, there are two options: