Thanks for the replies eveyone.
I don't imagine it could be old, unless it was deposited there not long before I found it.
It was buried where the sand meets solid reef, less than 100 meters from shore.
It was a few years ago now but i remember seeing the top exposed in 6-8 foot of water. Definitely in the impact zone in any decent swell so i would imagine that sand patch would be constantly moving and would have broken it if it was there for any length of time.