Swedish naturalist Anders Sparrman was Forster's assistant on this second voyage of James Cook, and he described the spotted shag in 1786 as Pelicanus punctatus.

In 1930, the Stewart Island population was described as a separate species, the blue shag (Stictocarbo steadi), distinguished by its narrow rather than broad white stripe on its head and neck.

[4] A 2014 molecular study by Kennedy and Spencer found that the spotted shag belongs to a clade of Old World cormorants.

Its closest relative is the Pitt shag (P. featherstoni), and their common ancestor split from a lineage giving rise to the black-faced, pied and little black cormorants of Australia.

Their bodies are very slim and they have a very distinctive black, decurved, double crest growing on their nape and their forehead.

They have a small patch of bare facial skin between their eyes and bill, which turns green-blue just before breeding season.

Sexual dimorphism means that the males and females of a species look different and can be easily told apart, which is not the case for the spotted shag.

[7] Large colonies can be found around Banks Peninsula near Christchurch, and in a rocky outcrop named the 'Shag Rock' in Wellington Harbour.

[11] This species prefers to nest on cliffs along rocky coasts where they occupy ledges or cracks, forming colonies of over a thousand birds.

[12] The spotted shag lays one to four (usually three) blue eggs with chalky covering, with both parents continuously guarding the chicks for about 30 days.

By this time, the tail is also fully grown and many birds will have left the nests to form small groups.

They form large winter flocks of up to 2000 birds, often flying in long lines between their feeding and roosting areas.

At this moment, gulls forthwith fly to the nest and standing on the edge of it, their whole manner and tone of voice convey the impression of swearing.

[6] The birds catch fish by diving from the sea surface and propelling themselves underwater with their webbed feet.

[7] Spotted shags often carry some small stones in their gizzard, which might function as to grind food or to avoid unwanted gut parasites.

[7] Spotted shags might be affected by the lice species Eidmanniella pellucida (Rudow, 1869) and Pectinopygus punctatus (Timmermann, 1964) (Pilgrim & Palma, 1982).

[19] DNA analysis has recently indicated that the birds originating from the northern parts of New Zealand are genetically distinct thus increasing the concern over the continuing decline of the species in and around Auckland.