[4] The blue devilfish is found in the western Pacific Ocean from north eastern Australia and New Guinea, through the Great Barrier Reef and the Coral Sea to New Caledonia.

[4] Assessor macneilli is found on reefs at depths of 2–20 metres (6.6–65.6 ft),[3] where it can group together in shoals of up 100 fishes in caves, crevices and under ledges,[4] where they often swim upside down.

[3] It is a plankton feeder and its main food are crustaceans such as copepods, ostracods and amphipods.

It is a mouthbrooder and the male guards the eggs in his mouth for 15–16 days, as hatching approaches the male keeps his mouth open continually and shifts the eggs backwards and forwards every 2–8 minutes.

Whitley from specimens collected at Hayman Island in the Whitsunday Passage off Queensland.