Derek Purdy was born in Remuera, Auckland, New Zealand on 26 December 1923, the son of Norman Henry and Elsie Mary Purdy (née Brickell).  In civilian life Derek Purdy was a grocer.

He joined the Royal New Zealand Navy Volunteer Reserve as an Able Seaman.  He was later selected for promotion to Officer and was loaned to the Royal Navy.  He volunteered for service in submarines and for ‘Special Service’ and was appointed to HMS VARBEL for X-Craft training.  He was promoted to Temporary Sub Lieutenant RNZNVR on 25 February 1944. After completion of his X-Craft training he joined HMS X24 as the First Lieutenant of the Operational Crew.

Whilst on tow by HMS SCEPTRE, in poor weather, for an attack on a large floating dock at Bergen (Operation HECKLE) Derek Purdy was lost overboard from the X24.  His Commanding Officer Lieutenant Pat (Percy) Westmacott had, reportedly. decided to tidy up the submarine before the attack and ditch the gash overboard. Derek Purdy went out of the Wet & Dry Compartment to do this and was washed overboard. He was replaced by a member of the passage crew and the attack proceeded and resulted in the sinking of the floating dock and a merchant vessel moored alongside it.  X24 managed to withdraw safely after the attack and was taken back in tow by HMS SCEPTRE for the passage back to Scotland.

Derek Purdy is commemorated on the New Zealand Naval War Memorial at Devonport, Auckland, North Shore City, New Zealand on Panel No. 7 and on the 12th Submarine Flotilla memorials at Rothesay on the Isle of Bute and at Kylesku.