Update 4 Jan 2019, 22:10:
The specimen has been identified by Dr Alastair Richardson as Engaeus leptorhynchus (see: https://twitter.com/nature_trackers/status/1213375015116128258)

We encountered this tiny freshwater burrowing crwayfish just off the Moon Valley Rim track, at the little track that leads to Gough's battery (a tin mining relic) Poimena, Blue Tier Regional Reserve.
Nearby is (what I've now learnt is) a man-made tailings dam.

I'm not sure if I've got the pin drop position quite right - but I'm confident of this verbal description of the location

Moon Valley Rim track details here: http://noloneliness.com/moon-valley-rim/

For a sense of scale: the length of my fingertip (distal phalanx) is 24 mm.