The box, which had belonged to his father's mother Catherine "Kate" Bell Hay, contained many letters and photographs from her time in New Zealand: 1912-1917 when she was assistant matron at Napier Girls' High School.
Most of the letters were from his grandfather Peter Hutchinson Wardlaw, whom Mark's father Elliot never met.
"Here was a chance in these letters to find out about my grandparents," Wardlaw told the Herald.
He'd unearthed a tragic love story broken by the war.
The subsequent book, Broken by Messines in WW1 -The Grandparents I Never Knew has been published to coincide with the centenary of Messines.
The family letters tell how Kate sailed from her home in Alloa, Scotland, on the SS Remuera to New Zealand in 1912 to work at Napier Girls' High.
Her engineer boyfriend Peter Wardlaw stayed behind to manufacture his seed-sowing machine in Lutterworth, England.
But soon after parting, Peter sent her an engagement ring.
Her plans to come home and marry him were thwarted by the outbreak of war in August 1914.
Peter volunteered for the Royal Field Artillery and was sent to Gallipoli, before the slaughterhouse of the Western Front.
Kate wrote more than 200 letters to Peter who witnessed many of the bloody battles at the Somme.
"These [letters] would have had a good psychological effect on him, lifting his mood in the trenches," Wardlaw said.
As the war dragged on, Peter struggled to maintain his spirits, which was reflected in his increasingly-less romantic correspondence to Kate 18,000km away in Napier.
In September 1916 he wrote: "I have just received your parcel and letter of July 2nd and take this opportunity of acknowledgement. Time is so jolly awkward now Kate for writing."
Wardlaw wonders whether Peter's love for Kate was fading into a distant memory.
"Could he still relate to the innocence in Kate's letters, with her joy in wood carving, socialising and travelling to exciting places?"
In December 1916, after almost five years Downunder, Kate risked the long sea voyage and threat of U-boats to return to Britain.
They had a "passionate but brief" reunion in Scotland in April 1917 before Peter had to return to prepare for the Battle of Messines.
Although the attack was a success, Peter was wounded and lost a leg.
While Peter convalesced at a London hospital where a prosthetic leg was fitted, Kate wrote with news she was pregnant. They married in September 1917.
But it was all too much for Peter, who deserted her.
"The war had changed him; Messines had broken him physically and emotionally," Wardlaw said.
Elliot Wardlaw was born in January 1918. His mother Kate died of heart failure in 1938, aged 50.
"My father was always quite sad as the calendar approached December. He would recount how he had held her in his arms as she died; the only parent he had ever known."
EVENTS MARKING THE CENTENARY OF THE BATTLES OF MESSINES
• A national commemoration to mark the heavy price paid by New Zealand troops at Messines 100 years ago will be streamed online today. The remembrance ceremony at Pukeahu National War Memorial Park in Wellington starts at 11 am and can be followed here ww100.govt.nz/messines-national-ceremony or here WW100NZ
• South Wairarapa District Council, Featherston Memorial RSA and the wider community are commemorating the centenary of the Battle of Messines at 1pm. About 60,000 New Zealanders trained at Featherston Military Camp before serving on European battlefields, including Messines, between 1916 and 1918. A wreath laying ceremony at Featherston War Memorial will be followed by a screening of the documentary March On at Featherston Memorial RSA.
• In Belgium, New Zealand will mark its involvement in the Battle of Messines with the National Commemorative Service at Messines Ridge British Cemetery at 8am and the Sunset Ceremony at the New Zealand Battlefield Memorial at 7.30pm.
