His grandfather, Norman Henry Prior, volunteered in 1915 at the age of 33 and set sail on October 9 as part of the NZ Medical Corps, landing at Gallipoli with the 7th Reinforcement.
He was there for the last two weeks of the campaign and was transferred to France with the Otago Regiment.
In the Battle of Messines and the Somme, Captain Prior earned a reputation for "devotion to duty in the field with little regard for personal risk". He was awarded the Military Cross for bravery relating to his actions on the night of September 15, 1916, searching for the wounded in "no-man's land" on the Somme.
It was the first day armoured vehicles (tanks) were used in warfare.
At his funeral in 1967, a veteran remembered Dr Prior as the man with a torch light attracting fire from both sides while looking for wounded.
Mr McLaren said this didn't stop him, however. "He realised that so he attached his torch to the end of a stick."
Dr Prior was wounded in 1917 at Messines but continued his service, which totalled more than three years.
On his return home he continued to practise medicine in the front of his Perry St home and died at 85 years old.
Mr McLaren said the Priors have just turned over 100 years in continuous practice in Masterton.
"My lasting image was getting an injection given by him, he didn't show the same skill that modern nurses have."
He remembered his grandfather as a "very slight man" but didn't recall him talking about his time overseas.
"None of them talked a lot about it, of course, which is understandable ... it would have been dreadful."
On one occasion, his grandfather had been sent to help 40 wounded men, laid out in the open.
Mr McLaren said it would be a particularly special Anzac Day for him and his son, as there were many connections to World War I in their family.
They were going not only to commemorate his grandfather but also a former neighbour who served. "He actually named his farm after Sedd el Bahr, a place in Gallipoli."
They also had family who served in Egypt, including a great aunt who was a nurse instrumental in setting up a hospital in Cairo.