Amphithalamus sundayensis is a species of minute sea snails, marine gastropod molluscs or micromolluscs in the family Anabathridae.
The length of the shell attains 1.2 mm, its diameter 0.6 mm.
(Original description) The shell is minute and ovate, with a blunt apex. It features four flattened whorls and an impressed suture. The aperture is detached, broadly elliptical, and slightly angled behind. The peristome is thickened, and a ridge connects its posterior end with the body whorl. The umbilical furrow is wide.
The whole shell appears quite smooth. Its colour is light olive-brown, with the suture, peristome, and lower part of the body whorl being darker.
This species is endemic to New Zealand and occurs off the Kermadec Islands.