Just as an impact on land can hurl ejecta for many miles, a large body landing in 1000 ft of water would eject huge volumes of water into the air in all directions. This enormous mass of water would crash back to earth some distance from the impact site, and may greatly amplify the tsunami that propagates along the surface.
In addition, the crater is on the edge of the continental shelf, and it may have caused a large underwater landslide which would also increase the strength of the tsunami.
Here is a map of the area. Notice the underwater shelf (southwest of the South Island) that drops off dramatically toward Australia to the northwest.
