"I was totally flabbergasted," Mr Christophers said. "Less than two weeks ago the phone call came out of the blue."
He said Cunard's tour organisers read a book called Poppy Boys by Invercargill writer Lynley Dear, which was based on the story of Mr Christophers' Southland grandfather Reginald Christophers and three of his brothers, who all died in World War I.
The family are not named in the book because Dear did not know there were any surviving relatives. But Mr Christophers read a review of it, recognised his family and contacted the author, so when Cunard asked her for a relative, she pointed them his way.
He will write a blog about the trip, which will take in Asia and the Gulf before sailing through the Suez Canal and up to Gallipoli.
He and his wife will then visit the grave of his great-uncle Victor Christophers, who is buried on the headland's North Beach. They will fly home on April 28 at Cunard's expense.
"I've always kept it in my mind as being something fairly special and felt I owed a debt of gratitude to those young men who died all those years ago," he said.
"I wondered what I was going to do this year and I thought I could go to Southland Boys' High School and be part of their commemorations, because they do it every year.
"So this is really quite amazing. I'm now desperately trying to get ready."
