That is a DHC Caribou. It was a STOL (short take-off and landing) tactical transport. That one is USAF - identifiable by the radome on the nose (our RAAF ones didn't have the radar system) and markings. USAF originally knocked them back, but poached them from the US Army after they bought them and proved their usefulness.
The RAAF (based at Phan Rang) flew them in Vietnam and for decades after as well. Once in 1968, I witnessed a RAAF pilot land and stop a fully loaded one in less the 500 feet, drop the load and take off from a dead stop in less than 100 feet. That was an amazingly skill-full pilot who had true feel for his aircraft.