Before deciding which design to build locally as the Wirraway, CAC were asked to import two different prototypes from North American Aviation.
Here’s a quick summary:
The NA-16-1A (charge code NA-32) was a fixed-gear trainer with a direct-drive Pratt & Whitney Wasp R-1340 driving a two-bladed propeller. The newspapers of the day commonly referred to it as the “NA-16” (a little misleading, since the NA-16 was a different aircraft, but they didn’t really understand that, and the name stuck).
The NA-16-2K (charge code NA-33) was a retractable-gear trainer with a geared Pratt & Whitney Wasp R-1340 driving a 3-bladed prop. It was commonly known as the “NA-33”.
The drawing below shows these two aircraft compared with the Mk I Wirraway.
The RAAF held a “fly-off” between the two aircraft and decided that the NA-16-2K was a more capable aircraft.
Therefore the Wirraway was based on the NA-16-2K, but with numerous modifications added to meet RAAF requirements.