For those that do not know what this snail looks like when it is alive, here is one. It was found in a small patch of sand in the midst of a Seagrass bed at Kohimarama Beach. This species is still recovering from the TBT pollution the caused imposex in various Neogastropods.
I am yet to find a living Alcithoe arabica to photograph, it too was badly affected by the TBT.

Amalda depressa has lacks the 'speckled' mantle that A. australis has and A. novaezelandiae has a much narrower build, is much smaller and tends to have a lighter colouration with the mantle having dark brown or almost black 'speckles'.
A. northlandica is a similar size to A. novaezelandiae, but has a slightly wider body whorl with a shorter more stump spire and the colours tend to be more pronounced.

Please correct me if I am wrong about anything.