New Zealand's involvement in the war had begun in 1939, and with Axis naval activity occurring in New Zealand waters, domestic defence plans were necessary, with much capital being invested. Matters became urgent with the entry of Japan into the war in 1941. A number of coastal defences were already in place throughout the country, with a concentration in Auckland, and Auckland City Council felt the need for air raid shelters in the central city, in preparation for Japanese attack. Work began in December 1941, and by January 1942, 16 300 feet of slit trenches had been dug around the city. Additional shelters were created in basements and old existing tunnels.