The aircraft departed Oodnadatta with 6 persons onboard. About 15 minutes into the flight, as it reached top of climb at 7000 feet, the fuel pressure to the right hand engine decreased and the engine back-fired. The pilot turned the fuel boost pump on but the engine failed to respond. The propeller was feathered. The aircraft was unable to maintain altitude and the pilot decided to return to Oodnadatta and advised Flight Service of his intentions. The landing at Oodnadatta had to be made on runway 13 as the other runway was closed due to soft, wet surface. Runway 13 had a crosswind component of about 15 knots at the time and the pilot experienced some difficulty in aligning the aircraft with the runway on final approach. Suffering from some apprehension, and concentrating on the approach and landing, the pilot neglected to extend the landing gear.