I think you might find that the ones with holes are re-using American style pins in the Australian position.
Australian pins are flat blades measuring 6.35 by 1.6 mm, 17.35 mm in length.
American pins are flat blades measuring 6.35 by 1.524 mm, 15.875–18.256 mm in length.
Interestingly, according to Wikipedia the hole doesn't really have a safety or electical function.
The small hole near the end of the power (non-ground) blades of some
NEMA plugs is used for convenience in manufacturing; if present, it
must be of specified diameter and position.1 Small specialized
padlocks are available to fit these holes, allowing "lockout" of
hazardous equipment, by physically preventing insertion of locked
plugs into a power receptacle.