[Update] – The source of the pumice is likely to be from the 2012 Havre Seamount eruption 800km NE of New Zealand. The eruption of this undersea volcano produced floating rafts of pumice estimated to be up 26,000 km²! Thanks for the info Matt!
These shelled creatures are known as Gooseneck barnacles. They were covering the recently washed up pumice on Elizabeth beach. Many of them were still alive, you could see them extending their fan like fronds.
Pumice forms in violent volcanic eruptions with explosive ejections of magma that cool so quickly that they solidify with fine bubbles of gas throughout. The resulting rock is of such low density that floats on water. The volcano that this pumice came from must have been a long way away, nothing like that near the east coast of Australia.