Anybody know the etymology of this one?

G'day joey, and welcome aboard,

I guess it's because most of our good old outside toilets (or dunnies, as we usually know them) were/are of somewhat flimsy construction--wood, fibro cement, and so on, and were easily damaged or even knocked down, as in the old Aussie saying: "May all your chooks turn into emus and kick your dunny down!"
Thus a strong, well built man would often be referred to as "built like a brick shit house"--implying a quite unusual, in fact unique quality.

Hope this helps,

Rgds, lusy (aka "rhapsody lute " by the spell checker)