2 pics of the one DEAD shell found during shallow snorkel in rocky inter-tidal.. Photos taken mainly because at first I thought it may be alive ( upright position despite significant tidal surge for such a precariously balanced object) but after imaging I nudged it and realized it was dead,nor was it home to a hermit crab. However such a distinctive shell may still attract a species ID on iNat so I now submit it 7 years later !
From memory it was ~6-8 cm long (thus about 3-4 cm tall including ridge). Sometimes a component in local shell grit or as almost intact beach-washed shells but not an especially common shell in my LGA anecdotally.