A 'lobster' (referenced when Darcy purchases the guns from his friend still in the army) is an Australian colloquial term used for the twenty dollar note whose distinctive red/orange colour is likened to a cooked lobster
The aircraft that they fly to the outback in is a Piper Chieftain, and actually manufactured and bought into service after the movie was set. That same aircraft is now used within an air ambulance service for flying patients between hospitals within Australia.