Shane Christie, a former Maori All Blacks player, had complained of headaches, depression and mood swings before his suspected suicide in 2025.
Pathologists who studied Christie's brain found signs of chronic traumatic encephalopathy, New Zealand media reported, a degenerative disease causing growing alarm in contact sports across the globe.
A University of Auckland pathologist found Christie had "high stage" CTE, noting he suffered "several significant concussions during the course of his playing career", according to public broadcaster Radio New Zealand.
Christie had campaigned before his death for more research into the effects of concussion on professional rugby players.
The former Super Rugby player donated his brain to the University of Auckland so it could be investigated for CTE, which cannot be detected in living people.
Christie's close friend Billy Guyton -- who died in 2023 -- was the first New Zealand professional player to be diagnosed with the disease.
New Zealand Rugby said it acknowledged Christie's results, Radio New Zealand reported.
World rugby's governing body is the subject of a legal case by around 300 players, who have accused administrators of turning a blind eye to the impacts of concussion.
Hundreds of American football players have been affected by CTE, which is linked to sometimes drastic behavioural changes.
A 2023 study by the Boston University CTE Center said that of 376 brains of former NFL players, 345 of them were found to have CTE.