William was captured by the Japanese in April 1942, in Indonesia. 

He was killed in the sinking of the Japanese ship Tamahoko Maru, torpedoed by the submarine USS Tang on 24th June 1944, in the Pacific, on route to Nagasaki. The ship contained 722 POWs of which 560 died.  It is worth noting that the Japanese did not mark POW transport ships, or inform the Allies that POWs were being transported.

"SULLIVAN.—CpI. William Bernard (Bill), who was drowned owing to torpedoing of ship on which he was being transported as prisoner of war from Java. Third beloved son of Mabel C. and late Charles T. Sullivan, care 14 Ferryhill Rd., Epsom, Auckland, late Lyall Bay, Wellington. Beloved brother of Cornelia, Frank, Nita, John, Carrie, Kathleen, and Jim. For the freedom we live for today." [Dominion, Volume 39, Issue 62, 6 December 1945, Page 1]