On enlistment at Trentham Camp on the 6 April 1916, he was working for the Public Works Department in Christchurch. Embarked per HMNZ Waitemata in Wellington 25 July 1916. Disembarked at Devonport and marched into Sling Camp 3 October 1916 WIA (Wounded In Action) 7 June 1917 and evacuated to the 77th Field Ambulance and then transferred to New Zealand Pioneer Battalion 3 July 1917. Evacuated sick 13 November 1917 and transferred to a number of hospitals before being admitted to the New Zealand Convalescent Hospital, Hornchurch. After discharge on 19 March 1918 he was transferred to the Rarotongan Detachment in Egypt. He moved to the 8th Rest Camp and then the 3rd New Zealand Depot Unit of Supply at Marseille for passage to Egypt on 16 May 1918. Joined the Rarotongan Company 15 June 1918, and returned to New Zealand on the SS Malta ill with malaria. Discharged 13 June 1919.

Information received from Sheryl Marsters May 2019: "I am the wife of the late Howard Marsters and have some info about William in addition to what you already have.
William is the son of Ann Marsters (Palmerston) & Raki Raki (Aitutaki).
William married a pakeha woman named Lillian Samuels whom he met when he was hospitalised on Somme Island after being injured in the war. She was a nurse there. They then settled in Pio Pio and William worked as a chef in Te Kuiti until his passing. The couple had no children. The photo of him buying bread in Belgium has been used in many places including the War Memorial Museum in Wellington. When Howard & I visited the museum & we saw the photo with William's name and cited him as a Maori soldier, Howard asked to speak to the curator to ensure he knew that William was one of the 500 Cook Island soldiers that served in the NZ Army. Hope that helps fill in some gaps."