While sinking, the sea floor is consumed by being melted back into the depths of the hot earth. High right and low left - sliding faults. The ridges and trenches are in heavy, sea-floor rock. The sliding faults are in lighter, continental rocks which will not go down subduction trenches. New Zealand is made of continental rocks.
Continental rock is light and floats on the heavy sea-floor rocks.
The shelf has considerable economic importance, as it is a popular place for fish. Much of the shelf came from erosion of Australian and Antarctic Mountains, before the Tasman sea and great southern oceans opened by sea-floor spreading, one of the reasons Australia only has mole hills for mountains. Australians point out, from their stable continental mass, that New Zealand has many geological faults , the most significant being it is above sea level. The only reason New Zealand is above sea level at all is elevation of the crust by collision of the Pacific Plate with Australia. Volcanoes and earthquakes are concentrated along the plate margins.