These were also discovered elsewhere back in the 1930's in Costa Rica.

http://www.ancientx.com/nm/anmviewer.asp?a=75

Giant Stone Balls of Costa Rica
Workmen hacking and burning their way through the dense jungle of Costa Rica to clear an area for banana plantations in the 1930s stumbled upon some incredible objects: dozens of stone balls, many of which were perfectly spherical. They varied in size from as small as a tennis ball to an astonishing 8 feet in diameter and weighing 16 tons! Although the great stone balls are clearly man-made, it is unknown who made them, for what purpose and, most puzzling, how they achieved such spherical precision.

Oooh, I like mysteries!

Formed 60 million years ago, discovered 1000 years ago, and guess what - they're still here! Look on http://www.aatravel.co.nz/101-must-dos-for-kiwis/Moeraki-Boulders.html