The attack went ahead and it was an unmitigated disaster. There wasn't nearly enough artillery fire to sweep away the barbed wire and many shells landed short amongst the Kiwis, causing hundreds of casualties. Those who survived were cut down by German machine gun and rifle fire as they advanced towards the German lines.
Thousands of families throughout New Zealand lost a brother, cousin, uncle or father.
Dr Macdonald came face to face with the pain and anguish the New Zealanders suffered on October 12 when he interviewed two survivors, who were then in their late 90s. Dr Macdonald was only a teenager.
"They talked about moving forward and the chattering of the German machine guns, of seeing the flashes of flare, shell bursts, machine gun muzzle flashes, seeing friends fall around them, one was wounded in the foot, the other went to ground to shelter.
"Both spent the day in shell holes with their friends either wounded or dead lying around them.
"That long wait after the initial attack was the worst part, because that mental focus they'd worked up to go into battle, for an attack that they knew was going to be a failure, had been broken and the grizzly reality of it was all around them.
"Having a look over the rim of a shell hole was to court death by a sniper's bullet, and one of these old guys I spoke to… one of his good friends was shot straight through the head by a sniper, and this old guy then had to spend several hours in that shell hole with his friend.
"It's a very humbling experience when you're 18 and sitting next to a man who's nearly 100, 80 years between us, but even in that length of time the grief, the post-traumatic stress of having seen all those things is seared into their minds.
"It was not uncommon for these old boys to sob quietly and confess to having nightmares about Passchendaele over the years.
"It's not just something in the history books for me, it's about people I knew, and that's really important to me."