In July 1915, Erwin enlisted in the New Zealand Army Nursing Service and was posted overseas. In October that year, she was aboard the SS Marquette when it was torpedoed by a German submarine and sunk. Ten New Zealand nurses lost their lives, but Erwin and other survivors continued to serve in hospitals and hospital ships for the remainder of World War I. After the war had ended, she continued as a nurse in England before returning to New Zealand in 1920.
