Adult A. granulatus can be distinguished from other Amychus species by its wide and spreading prothorax and small round nodules on its elytra.

[5] This species was discovered by the lighthouse keeper Andreas Sandager on North Brother Island in Cook Strait, and was described by Broun in 1881.

[3][6] It is currently only found on islands in the outer Marlborough Sounds in New Zealand;[6] fragments of this beetle have been collected from the nest of the extinct Laughing owl in North Canterbury, indicating it once had a much larger former range.

[6] The adult beetle has been collected from tree trunks including tawa, ngaio and taupata at night, and from under logs and rocks.

Providing artificial habitat to replace the lack of decomposing logs may assist this species and help with population monitoring.