But before Mr Pirihi committed to being a part of the documentary he visited his uncle, Grant Pirihi, who is the son of Ngawaka Pirihi, to get his blessing.
"I was scared because first and foremost I didn't know how he would react. I put it to him and I said 'Uncle Grant, I would like to show Aotearoa what our tupuna did,' and he said 'I trust your judgment.'"
Mr Pirihi learned about his great uncle through public records and stories his Uncle Grant shared with him. He says it was a very "moving" experience learning about his great uncle and what he endured during the war.
"I feel a sense of pride, a sense of mana. He didn't go to fight, he went to help young tane.
"My Uncle Grant said Matua Ngawaka used to have 'me-time' and Uncle Grant would ask him what he was thinking about. He only told quarter of the story. Some stuff he told of was young tane coming into makeshift medical tents with limbs hanging off or no limbs at all. It's hard imagining that happening in the life we live now. I wouldn't wish that upon my worst enemy."
As part of the documentary Mr Pirihi and Mr Tarr gave blood as a reflection of the blood their ancestors gave when they served at war. When Mr Pirihi first met Mr Tarr he thought he was part of the crew, but as they sat next to each other while giving blood they began talking about their relatives who served in WWI and Mr Pirihi quickly realised he was also part of the documentary.
"His great grandfather, Reginald John Claude Jellie, was a sergeant in the New Zealand Army. We found out that his great grandfather also enlisted at age 20, like my great uncle. It was really nice sharing stories," he says.
The pair discovered there were similarities between their tupuna - they both served in the Battle of Somme and they both suffered shrapnel wounds. He says the pair both wondered whether their relatives had met and if Ngawaka had ever helped Claude.
But overall Mr Pirihi says his new found knowledge of his great uncle means when he stands at the Dawn Service on Monday, he will be holding his head high feeling pride and mana knowing about his great uncle.
"I was contemplating this. I had not been to a Dawn Parade in a number of years until I went last year for the centenary (of Gallipoli landings). I think it's taken on more significance for me. Just knowing the things he did, it is important to honour the men and women who served in any of the wars. I have a feeling of appreciation. I don't know how I would have done it."
Mr Pirihi said he would be going down to this year's Dawn Service at Auckland War Memorial Museum with his son.
-The documentary can be viewed at www.maoritelevision.com/tv/shows/anzac-2016/S01E001/blood-we-share.