Onchidina is a monotypic genus of air-breathing sea slugs, a shell-less marine pulmonate gastropod mollusks in the family Onchidiidae.

[2] Uniquely within the Onchidiidae, O. australis lives above the tidal zone in a fully terrestrial habitat.

The dorsal mantle surface exhibits strong, irregular granulation, lacking any evidence of pseudobranchiae.

Two more or less indistinct bands of spots traverse the dorsal surface longitudinally, maintaining a consistent distance from each other and from the mantle margin.

The ventral surfaces of the foot and head display a yellowish or reddish-gray coloration.