Pill millipedes – order Sphaerotheriida
Whilst these resemble slaters (isopods) superficially, they're actually a myriapod rather than a crustacean. They're not closely related at all.
Pill millipedes behave like slaters (isopods) by rolling into a sphere when disturbed. This gives them their name. New Zealand pill millipedes are members of the order Sphaerotheriida, a group with a Gondwanan distribution, with ~100 species known from Australasia, South Africa, and Madagascar.
NZ Nature Photos