In the past, oceans covered most of the rocks and earth that we see today . This is both from the land being lower and oceans levels being higher. over millions of years the plates have shifted resulting in areas of land becoming more exposed and some areas of land becoming inundated with water.

New Zealand straddles the boundary between the Pacific and Australian tectonic plates, leading to significant geological activity. This boundary is marked by the Hikurangi subduction zone off the East Coast of the North Island and the Alpine Fault in the South Island. The Pacific plate subducts beneath the Australian plate in the North Island, while the Australian plate subducts beneath the Pacific plate further south. The Alpine Fault represents a transform boundary where the plates slide horizontally past each other 

• Formation:
  The Hauraki Gulf was formed by rising sea levels that inundated a broad coastal plain after the last ice age.

• Shallow Water:
  The Hauraki Gulf's water depths vary considerably. Inner Gulf areas, south of Tiritiri Matangi Island, typically have depths less than 40 meters. The central Gulf, between Tiritiri Matangi Island and Cape Rodney, has depths between 40 and 50 meters. As you move north, towards the Hen and Chicken Islands, depths increase to between 50 and 100 meters. About 3.4% of the Hauraki Gulf Marine Park is deeper than 200 meters.  

• New Zealand sits atop the submerged continent of Zealandia, This means that large portions of the coastline are relatively shallow for a great distance offshore in comparison to other continents

• Islands:
  Many islands are remnants of coastal hills that were once part of the landmass before the sea levels rose. 

• Harbors and Estuaries:
  River valleys were transformed into harbors and estuaries, creating a network of sheltered waterways. 

• Volcanoes:
  Two of the youngest volcanoes in the Auckland region, Rangitoto and Motukorea (Browns Island), are located in the Hauraki Gulf. 

• Greywacke Rocks:
  Many of the islands are composed of greywacke rocks, a type of sedimentary rock that is also found in the Hunua Ranges and land south of Port Waikato.