The first stop for Johnston was "Sling", the New Zealand military camp on Salisbury Plain. Johnston's troop was sent towards the heavily fortified high ground east of Ypres, to push through to the Belgian coast.
As winter neared, the battleground became pitted by incessant shelling. Ormond Brunton's History of the Auckland Regiment records, "The beautiful woods were dead, horribly dead [and] there was no grass or pleasant herbs. The streams ... were horrible bogs. No bird sang."
At Westhoek on January 29, 1918, Johnston was either directly shelled or driven out of his trench by a shell and then shot.
Johnston's left arm and right leg were shattered. His best mate - and best man - Ian Hutchison carried him to safety. Hospital was weeks away.
Johnston first had to go to a casualty clearing station, then the NZ Field ambulance in February. Two weeks later, he was admitted to hospital at Etaples, then transferred to a hospital ship in March.
Finally, Johnston arrived in England, where his arm was cut off. He was one of 21,000 treated at Brockenhurst.
In April 1919, Johnston picked up the Spanish Flu sweeping around the world. That almost killed him - but he was too staunch.
Nursed back to health by English volunteers, Johnston finally returned to Tauranga in July.
Life for "wingies", as the armless amputees were known, was a mixture of good and bad. Nine hundred and sixty wingies and legless limbies were sent home from the NZEF - 345 had had their hands and arms cut off.
Johnston came back to a country where Limbless Soldiers Picnics continued well into the 1930s. Johnston resisted the Depression by partnering in a successful plumbing business.
Brian McCord was the 16-year-old Tauranga boy chosen to drive Johnston home from the plumbing business every night.
"I was proud of taking him home but he never spoke about the war," McCord remembers.
He's now 80, but he'll never forget the "wingie" carrying the collection plate around his Presbyterian church with one arm.
"He never appeared resentful about his missing limb, never mentioned it. Those military guys were very upright like that. He would swing his good arm in a military fashion as he walked."
Bert Johnston died in 1960 and is on the Katikati Roll of Honour.