The unique character of Lyttelton Harbour is reflected in the fact that breakwaters were required to protect the original port development which is nine kilometres from the open sea. The sides of the harbour, in part drowned marine cliffs, drop sharply to a relatively flat bottom. Waves passing up the harbour are consequently little affected by refraction, the greatest energy attenuation being caused by bottom friction. The extension of the port area and the additional dredging required to accommodate larger ships has necessitated studies of the harbour regime to determine the factors influencing wave energy and sediment transport in the harbour. This paper outlines the geological history of the harbour, which is responsible for its unique character, and presents the results of investigations carried out to date. It is concluded that the tidal currents are the primary sediment transporting agents, despite their low but sustained velocities, and that the principal determinant of sediment mobility is wave action. The words 'Regime Improvement1 in the title refer to efforts made to reduce wave energy in the harbour by utilising dredge dumpings to refract incoming waves. Reduction of wave energy in the harbour has as its twin concomitants the improvement of cargo handling conditions at Cashin Quay and a reduction in the dredging required to maintain the channel.