"Dunny" is Australian slang for a toilet. In this case an outhouse toilet that in earlier times meant a trip down the backyard to share the toilet seat with various species of spider whose joy in life was biting people on the bum….
They backed onto a lane from where, in pre sewerage system times, a waste removal contractor would regularly empty the contents of the bin. Later, they were connected to a septic system and later still they were connected to a deep sewer system. These later versions did not have the earlier access hatch at the rear. Many of these cute structures attract varying qualities of artwork/ graffiti.