A letter from Lieutenant Charles Hamilton Loughnan describes the "bloodiest bit of fighting in history" at Gallipoli in 1915.
The former New Zealander travelled to England in 1914 to study law but arrived to find war had broken out, his grandson Bill Werry said.
Lieutenant Loughnan enlisted in the English army and became a forward observer in the Royal Field Artillery, directing the fire onto designated targets.
He landed on the shores of Gallipoli in 1915 and took up defence positions with the New Zealand and Australian Army Corps.
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The 21-year-old's letter details "a night's desperate fighting" on August 5 when the troops succeeded in taking the "central heights" of the Gallipoli Peninsula in one great rush.
"I say glibly, in one great rush, but it was a whole night's desperate fighting and cost I dare not say what?"
Lieutenant Loughnan went on to fight the remainder of the war and was awarded the Military Cross in Ypres in 1918 before returning to New Zealand.
He was wounded by shrapnel in late 1918 and, after chest complications almost 50 years later, doctors found a fragment of German steel next to his lungs.
Lieutenant Loughnan lived in Rotorua until he died at the age of 84.
Read his full letter here: