HMS Sirius was the flagship of the First Fleet, which set out from Portsmouth, England, in 1787 to establish the first European colony in New South Wales, Australia. Sirius was wrecked off the coast of Norfolk Island in the Pacific Ocean in 1790.
The anchor,together with a cannon from the ship, were placed in the park in 1917.
http://en.wikipedia.org/wiki/File:Unveiling_of_HMS_Sirius%27_anchor.jpg