From the beginning of WWII it was anticipated that direct naval threats to New Zealand would be from enemy mines, and to a lesser extent, submarines. With the entry of Japan into the war in 1941, the threat of invasion also had to be considered.
From June 1940 disguised German raiders were active in New Zealand waters. ACHILLES, and later MONOWAI, as well as various Australian cruisers, steamed many miles in an effort to intercept the enemy warships. Four merchant ships were sunk off our coast and another eight within the wider Naval Station.
The urgent need to supplement the few ships in commission was met initially by pressing merchant shipping, fishing boats and private launches into service. Minesweeping was the immediate priority. As a precaution several trawlers were quickly requisitioned and fitted out as minesweepers to supplement New Zealand's sole minesweeper. They returned to their owners until they were required. Twelve minesweeping trawlers of the Castle Class were built in New Zealand, and four of the Scottish Isles Class, built in Britain.
Mines were also laid as a defensive measure off a number of ports and harbours. Sweeping these commenced in 1943 and continued until mid 1946.
To patrol the coast, private pleasure launches were formed into the Naval Auxiliary Patrol Service. They were based at Whangarei, Auckland, Wellington and Lyttelton. The service was disbanded in 1943 when Harbour Defence Motor Launches and Fairmile patrol vessels became operational. From early 1944 the Fairmiles deployed to the Solomon Islands area where they carried out anti-submarine patrol and escort duties.
The largest conversion was the Union Steam Ship Company's Liner MONOWAI, 10,850 tons gross register, which was fitted out as an armed merchant cruiser and commissioned on 30th August 1940. For the next two and half years she was employed on patrol, escort and transport duties in the South Pacific, finally seeing action in Fijian waters against the Japanese submarine I-20 in January 1942. MONOWAI paid off in Britain at the end of June 1943, and then re-commissioned as a landing ship. She took part in the Normandy landings of 1944.