The centenary of one of the greatest tragedies of the First World War will be commemorated by hundreds of New Zealanders and Australians early this month.

One of New Zealand's foremost war historians, Professor Glyn Harper, leaves this week to revisit Passchendaele where 846 Kiwi soldiers were killed in two hours in a futile attempt to take the Passchendaele ridge.

Of particular interest will be the extensive tunnels at Arras built by an independent tunnelling unit comprising up to 500 miners from the West Coast, Buller and Waihi.

'They built a network of tunnels over a year which contained an underground hospital, railway, barracks, rooms, military headquarters and dining rooms for about 20,000 troops,” says Glyn. 'In the tunnels, troops were moved forward to prepare for the Battle of Arras and, when the attack started in April 1917, they broke through the surface close to the German lines.”

Glyn, Professor of War Studies at Massey University, will spend two days on reconnaissance at Passchendaele and pay his own tribute to the battle which was ‘the darkest day in New Zealand military history', an attack that should never have gone ahead.

In Paris on October 6 he'll catch up with a party of New Zealanders who he'll mentor and act as their expert historian on their journey to the Passchendaele centenary on October 12.

Glyn, who wrote the definitive book about the battle entitled Massacre At Passchendaele: The New Zealand Story, says the attack, ordered by overall commander, Field Marshal Haig, was New Zealand's worst military disaster and should never have happened.

Passchendaele was part of the Ypres campaign which lasted from July to November 1917 and, early in October, four Anzac divisions succeeded in securing the foothills around the Passchendaele ridge and deserted village.

'It was a stunning success by the divisions, three Australian and one New Zealand, who attacked side by side. But, as they took the foothills, the rain bucketed down and turned the whole area into a quagmire,” he says. 'The divisions should have stayed until conditions improved. The weather was atrocious and it was impossible to move the artillery forward.”

Instead Field Marshal Haig wanted to continue and ordered the New Zealand divisions to storm 3,000 yards and take the Passchendaele ridge without adequate artillery support.

'It was an incredible distance in a quagmire and they were massacred. The ridge was well defended and the Germans knew in advance because a British deserter told them the night before.”

Within two hours 846 Kiwi soldiers were killed, 138 died of their wounds in the next few days and a further 2,000 were wounded. The Australians suffered similar casualties.

Our troops, commanded by General Andrew Russell, led the charge. It was the first time a New Zealand division had failed to take its objective. When the attack was called off that afternoon, it took three days to clear the battlefield.

Glyn says our division was a spent force and they saw little action until they played a significant role in winning the war in 1918.

'Their loss impacted on families throughout the country for decades to come. It shaped New Zealand as a nation.”

Many lessons were learned from Passchendaele. 'We learned that good quality soldiers were not a resource to be squandered and it was a betrayal of their trust that they were given an order that was impossible to fulfil.”

'It led to the Freyberg Charter in the Second World War where our government gave New Zealand commanders the authority to say no if they didn't agree with given orders.”