The soldiers' bodies and many wounded were left on the battlefield in no man's land - unable to be recovered and buried.
Although a temporary truce had been made with the Germans to allow the wounded to be rescued it was vetoed by senior officers, and the New Zealand troops were deeply troubled by their inability to recover and bury their comrades.
Bishop Cleary and an officer crawled out and lay in the snow amid the remains of the dead. In his diaries, Bishop Cleary comments several times on the dead lying "out there" and how the Germans used to shoot burial parties.
Just 50m from the enemy line he said a "De Profundis" over the bodies - a psalm which normally forms part of the prayers for the dead recited at Catholic funerals.
However, Bishop Cleary's wooden crucifix was badly damaged while he was in the trenches so the Kiwi soldiers gathered up spent shell cases and cobbled together a new crucifix for him.
The Catholic Diocese of Auckland said the bishop made himself a reputation as a scholar after the war and was a long-serving editor of the New Zealand Tablet before being appointed 6th Bishop of Auckland.
"Historical accounts also reveal Bishop Cleary learned Te Reo Maori, made arduous visits to remote rural communities, and took a special interest in the education and care of disadvantaged children," said spokeswoman Lyndsay Freer.
Returning to his duties as Bishop of Auckland in October 1917, he became a founder member of the RSA, and opened the doors of several Catholic schools to serve as hospitals during the flu epidemic.
"Bishop Cleary's crucifix and its welded bullet casings stand as a reminder of unselfish service given for God and country by an outstanding man who had little regard for his own personal safety and wellbeing in the heat of battle," Ms Freer said.