Hi mat I found similar pupal cases in the Mt Majura nature park east of the Fair mid of April this year and in March 2015 - there were 100's of them. Perhaps this is Trictena atripalpis, the rain moth, also known as bardee, bardy, bardi grub, or waikerie, a member of the Ghost or Swift Moth (Hepialidae) family. The adult moth have a wingspan of 12cm (male) to 16cm (female), the larvae feed on roots of Eucalyptus sp. Further info see http://lepidoptera.butterflyhouse.com.au/hepi/atripal.html
Correction: I took 2 photos of pupa cases on the 28th April 2015; from memory I noticed 100's of pupa cases 2 weeks earlier ie mid April 2015 which is around the same time as this year.
Thanks Waltraud - I'm also pretty sure that it is a bardi grub (a favoured bait of those fishing the Murray). CNM isn't including moths yet but hope to in the future