"It's unreal, it's devastating. When he arrived his oxygen stats were so low ... he turned into pulmonary edema where he was getting no oxygen," she told the Herald.
"He went into a cardiac arrhythmia but at first it looked like he was getting better.
"It was about 4pm [on February 7 local time] when he got into big trouble, at 6pm he coded and at 7.30pm they couldn't do anymore."
She, Clark and Byron had climbed other mountains in the past including Mt Egmont three times.
Mt Kilimanjaro had been a dream of Clark's for more than 20 years, Lisa said, one which takes nine days, six to climb and three to decline.
On their way to the summit, the 57-year-old became ill at 3000m but Clark told his son to continue while he waited there.
However, by the time Byron had summited and begun to climb down, Clark had been removed from the mountain and taken to the hospital where he later died.
Clark met his second wife Lisa in 1998 through an ancient history website when she was based in her hometown of Boston, Massachusetts.
The pair were both writing about Roman history when they connected, corresponding for a year before Lisa moved to New Zealand in October 1999.
Brett Clark at Nimes, France for the Grand Roman Games in April 2018. Photo / Lisa Clark
Clark was in the process of finishing his master's degree at the University of Auckland after completing a bachelor of arts with first class honours in ancient history.
He was the club captain of Imperium-Romana, a New Zealand organisation devoted to re-enacting Roman living history displays.
"We were both very active [with the group] and we did a lot, we did a lot of parades and did the Armistice in Cambridge every year," Lisa said.
"We've got a shed where we store 20 shields, four tents and four ballistas (catapults).
