Gravity measurements suggest that the earth's crust has a thickness of about 20 km beneath the south-eastern margin of the New Caledonia Basin. From a point 50 km within the edge of the New Zealand continental shelf the crust thickens to a maximum value of about 36 km near the Marlborough Sounds. Residual gravity and magnetic anomalies indicate that a broad belt of dense, magnetic rocks underlies the South Taranaki Bight but does not appear to extend beneath Taranaki Province. These rocks may be either Paleozoic intrusives similar to those found in the South Island, Or Pleistocene volcanics similar to Egmont Andesite.