Specimens collected from Spencer Gulf in South Australia have mantle lengths and weights "rarely exceeding" 125 mm and 521 grams respectively.

In 2015, South Australian Research and Development Institute (SARDI) scientist Mike Steer further described the species' cuttlebone as follows: "Sepions are elongate oval, acutely narrowing at both ends.

The posterior dorsal surface typically has a pinkish tinge and is covered with fine denticulate projections that diminish anteriorly.

"[5] Fishery dependent and independent data from Spencer Gulf has shown that largest populations of sexually immature Sepia novaehollandiae are caught between February and May annually.

Morphological similarities and overlapping geographical ranges have led to difficulties distinguishing S. novaehollandiae from juvenile Sepia apama in some instances.