Think Stinger sales are fine (but still, could be better) relative to what its competitive set actually is, rather than what Kia Australia tried to market it (at least initially) - the successor to the previous gen RWD Commodore.
Taking a look at (June) sales at various entry-level/compact (midsize for Australia) sedans and fastbacks...
Q50 - 27
4 Series GC - 29
A5 Sportback - 62
A4 - 111
IS - 121
Stinger - 200
3 Series - 339
C Class - 708
Only the C Class (which includes sales of the estate) far outpaced that of the Stinger.
Sure, the Stinger had a lower price-point, but a big part of that is not being sold under a lux brand and all the lux service amenities that entails (which is the reason why the
G70 costs a good bit more than the Stinger).
Pretty much all Stinger sold in Australia are for the V6 and in the upper trims - which is about where the IS starts in price (wouldn't be surprised if, like elsewhere, the majority of IS sales is for the base engine, along w/ the hybrid).
Where the Stinger could do better is in sales of its base T4 mill, but afraid that such a thing won't happen until the base engine gets upgraded to the 2.5T (a hybrid variant tied to the T4 also wouldn't hurt).