There are two subspecies, M. m. maxima, found in open country, and M. m. gigantea in the rainforest. The forest race is darker, less spotted above, and more barred below than maxima, but the two forms intergrade along the forest edge zone.
Breeding is from August to January, 3–5 eggs being laid in a riverbank tunnel.
Giant Kingfisher is 42–48 cm long, with a large crest and finely spotted white on black upperparts. The male has a chestnut breast band and otherwise white underparts with dark flank barring, and the female has a white-spotted black breast band and chestnut belly.
The call is a loud wak wak wak.