Astrpyga radiata or radiant sea urchin.

These guys exist in the warmer waters through out most of the indian and pacific oceans, in Australia, from shark bay in WA around the top to Moreton bay in Queensland..
They are found at all depths the average diver can reach, but quite often in less then one metre of water.
As with most urchins, they feed on most algae’s and small crustaceans.