Adult size is typically from 13 to 25 cm. Body is soft, and colour varies from light mottled brown to black. Tube feet are present on the belly extending up the whole body. Tentacles with disc shaped tips surround the slit-like mouth. On the back, A. mollis has pointed papillae (pimples) which can be reduced on preservation

The species is principally found at low-tide level on sand, as well as in mud in protected areas. Their vertical range goes from shallows waters on the coast to around 200 metres of depth

Described as detritivores, A. mollis mainly capture organic particulate matter from the substrata, which is highly nutritious.

Reproductive season is from October to February, and periodicity of spawning is regulated by lunar cycles, specifically following a full moon. Like other holothurians, A. mollis has the ability to regenerate its internal organs after undergoing a phenomenon called auto-evisceration. This process, which is thought to be induced by external stimuli, is characterized by violent and rapid expulsion of organs, followed by regeneration, which occurs at a slower rate than in other genera. Nevertheless, a more recent study reports no spontaneous evisceration or seasonal atrophy of the viscera, highlighting that handling these organisms does not trigger such reactions and even suggesting than during predation it would not occur easily. The scarcity of knowledge regarding A. mollis biology and ecology has hindered the development of a stable fishery industry.

Morgan, A. & Archer, J. 1999. Overview: Aspects of sea cucumber industry research and development in the South Pacific. SPC Beche-de mer Information Bulletin. 12. Slater, M.J. & Jeffs, A.G. 2010. Do benthic sediment characteristics explain the distribution of juveniles of the deposit-feeding sea cucumber Australostichopus mollis?. J. Sea Res. doi:10.1016/j.seares.2010.03.005 Sewell, M.A. 1990. Aspects of the ecology of Stichopus mollis (Echinodermata: Holothuroidea) in north-eastern New Zealand. NZ J. Mar. Freshw. Res. 24: 97–103. Slater, M.J. & Carton, A. G. 2009. Effect of sea cucumber (Australostichopus mollis) grazing on coastal sediments impacted by mussel farm deposition. Marine Pollution Bulletin. 58: 1123–1129.