About 80 million years ago the Tasman Sea formed as New Zealand drifted away from Australia, and Gondwanaland broke apart, stranding native ancestral species here, such as mosses, ferns, podocarps (ancestral kahiketea/ etc.), tuatara, frogs, ratites (kiwi/moa), weta, peripatus, two species of dinosaur (which soon died out), but no land mammals except bats. This split left New Zealand to evolve its unique ecosystem in isolation.
The Auckland area was land during this period, but had eroded away by about 30 million years ago.
Ancestral Auckland was thrust from the sea along with the rest of New Zealand 120 million years ago to become a mountainous strip along the coast of Gondwanaland.
250 million years ago Auckland's oldest rocks formed off the coast of Gondwanaland, and although you can't see any in Green Bay, they're the bedrock deep beneath. They're visible on the eastern side of the Auckland region, where they've been exposed through more recent erosion.
