Considering poor navigation knowledge it would be a fair guess that the skull is either Dutch (considering Batavia ((Jakarta)) was a hub of trade for centuries and is just NW of Australia, or Spanish considering the volume of ships attempting to get to and return from South America and all that gold.
In the 40's a rudder from a 16th century vessel was dredged up in this area by sand miners. Actually I saw it laying near the beach in the 70's until the local council dumped it not realising its importance! Also an aboriginal "clever man" in this area had a Spanish cutless when whites first settled here. He refused to say where he got it. Local legend says there is a wreck in the dunes ( and yes ground radar did pick up an image when the story broke in the 80's but was never investigated as too expensive) and that an old rock wall nearby is the remains of a "fort" created by the ship wrecked sailors.