I think this starfish may be a linckia laevigata, but I really have no idea. It is simply beautiful. If you look closely, you will see that its skin is mottled, with lighter patches of blue separated by darker blue lines.
I spent hours poking around rockpools in my childhood, until I was well through my teens, at Kawaroa in New Plymouth where I grew up, and at Port Jackson at the top of the Coromandel Peninsula, where my parents took us camping every year. When we stayed on Kangaroo Island earlier this year, the girls and I spent a couple of highly satisfactory mornings poking around rock pools there. We didn’t find any gloriously blue starfish, but we did find crabs and snails and sea anemones and seaweed and limpets and some lovely soft salmony pinky red starfish.