In 1942, during WW2, when fears of a Japanese invasion were at their height, an emplacement was built on Kaiti Hill. In September that year a five-inch gun was placed there, however, ammunition did not arrive until January 1943. The gun was originally manned 24 hours a day by members of the 77th Heavy Battery who slept at Te Poho o Rawiri Marae until an accommodation block was built. The gun was only operational during 1943 and 1944 before being dismantled.
Weeks after that on January 15, 1945 a German submarine U-862 entered Poverty Bay and, under cover of darkness, crept into Gisborne Harbour around midnight. By this time, most of Gisborne's defences, including the gun, the RNZAF airbase at Darton Field (now Gisborne Airport) and the 6.25 tons of barbed wire on Gisborne beaches were no longer operational or had been removed. The Germans observed locals promenading along the beach and the commander of the submarine saw no target worth sinking, so he left the harbour and sailed on to Hawkes Bay.
Further details of this unusual occurence can be found in this article.