This strange form is the egg case from the Port Jackson Shark.
It forms this twisted spiraling shape once dried.
The juvenile shark escapes from the far end, leaving an opening in the top
I found a number of them on Blenheim Beach (Marine Bay), located near Jarvis Bay area within the marine park.
Pictured with the fine white sand as a backdrop below
The Marine Park was established some time ago, as the area was recognized as a significant bio-diverse region warranting protection.
Along with the casings found, a number of sea slugs were found nearby amongst sea weed and a dragonfly nymph located in the fresh water of the creek within 25 meters of the surf.
While we enjoyed the surf, a pod of dolphins cruised close by.
A very pleasant and diverse place indeed.