Although the war seemed remote, German Raiders laid mines around New Zealand and captured some merchant vessels. In June 1917 the armed merchant raider WOLF laid 35 mines off Cape Farewell at the western entrance to Cook Strait, and 25 between North Cape and Three Kings Islands. A number of mines were swept, while others broke adrift and were washed up on the coast. However, two merchant ships sank after striking some of these mines, the SS PORT KEMBLA off Cape Farewell in 1917, and the SS WIMMERA between Three Kings and North Cape in 1918.
To sweep the mines, three trawlers, NORA NIVEN, HANANUI II and SIMPLON, were chartered and fitted for minesweeping in 1918. Two naval personnel in each vessel supplemented their normal crews. In June 1919, three RN Flower-Class sloops came to New Zealand to conduct check sweeps of the mined areas.