Oh that's easy then, it must be the Coriolis effect.
Perhaps it has to do with the original human invasion route taken?
Let's see, North America via Alaska, down the west coast, so that's driving on the right.
Genhkis Khan etc invaded Europe from the North East, so again they were driving on the right.
The Uk was probably settled via land bridge from France, and they'd have been inclined to go westawrds to stay with the good weather, so that's driving on the left.
In Australia they'd have landed somewhere near the middle of the top, and spread out in all the equally inhospitable directions, so they'd drive any old side of the road.
Well, it works in 4 out of 4 cases, so it must be right!
Cheers
Greg Locock