Guyton, who also played for the Wellington Hurricanes and Canterbury Crusaders in Super Rugby, died suddenly last year at the age of 33.
CTE, a degenerative brain disease linked to repeated head impacts in contact sport, can only be diagnosed when the brain is examined after death.
Radio New Zealand said Guyton's family had received the CTE diagnosis this week after testing by Auckland's Neurological Foundation Human Brain Bank.
"It was noted as CTE by the New Zealand-based pathologist," the Brain Bank's Co-Director, Professor Maurice Curtis, told RNZ.
"It was sent to an Australian pathologist for a second opinion."
The Brain Bank was unable to provide immediate comment when contacted by Reuters on Thursday.
The sport's national governing body, New Zealand Rugby, said it shared the Guyton family's concern at his diagnosis.
"NZR is concerned about the possibility that repeated head impacts during participation in rugby may contribute to neurodegenerative diseases in later life," it said in a statement.
Guyton retired from rugby at the age of 28 in 2018 after suffering repeated episodes of concussion from head knocks. He told a New Zealand media outlet everyday activities would trigger concussion symptoms.
"Watching TV would bring on headaches, doing too many tasks, loud noises, some days I would need noise-cancelling headphones or I would feel nauseous and have blurry or double vision, it was not very fun," he told the Nelson Weekly at the time of his retirement.
Guyton's father told RNZ his son had suffered before his death.
"The poor guy would spend hours in a small, dark cupboard because he couldn't handle being in the light," John Guyton said.
"Some mornings he'd just sit in the bottom of his shower tray crying, trying to muster up the energy to get moving."
Just under 300 former rugby players are suing World Rugby, England's Rugby Football Union and the Welsh Rugby Union over neurological injures, alleging the bodies failed to protect the health and safety of players.
The governing bodies say player welfare is the sport's number one priority and they will be guided by the latest science.
Rugby authorities have introduced smart mouthguard technology for head impact assessments and started trials to lower tackle height in community rugby as part of efforts to reduce the risk of head injuries in the sport.
