New Zealand sits on the junction of two tectonic plates - the Pacific Plate and the Australian Plate. Over a few million years there has been a substantial movement of continental crust along the Alpine Fault plate boundary amounting to a horizontal movement of 400km and vertical movement of 10,000 m with only 3000 m of the vertical movement remaining as mountains which means that the mountains fritter away rapidly, forming piles of stones called scree down the flanks of the mountains.
These piles of rock debris are carried away by glaciers and pebble–choked braided rivers to the sea. Pebbles get rounded by rocks colliding with each other. Despite the incredible rain fall of about 18m per year in such regions, the rivers appear not to have enough water and they are so 'congested' with rocks and pebbles forcing them to create multiple small channels called braiding.
The sea sorts the pebbles by shape and size accumulating flat, rounded pebbles along the length of the beach near river mouths, whereas smaller pebbles are displaced further away along the beach.
Another interesting feature of the 10 000m vertical movement along the plate boundary (Alpine Fault) is that deeper crystal metamorphic rocks have been brought to the surface and weathered. This has allowed rocks full of interestingly coloured materials rise to the surface, resulting in greywacke, black argillites, black marbles, yellow iron-stained quartzite, white quartzite, green sub-schist and greenstone (pounamu) among many others for pebble creation.