Australian infantry during the Battle of Aspern Essling (I note you did not specify troops actually AT the battle), would, I think, be The New South Wales Corps.

To model them you should use all of the discarded figures from your various Napoleonic British sets. The ones you threw in the bits box because you thought they were no good and you didn't want them to be part of your regular British battalions.
This should give you a fairly accurate representation of the Rum Corps.