A New Zealander who died last year is the country's first known professional rugby player to be diagnosed with the degenerative brain disease chronic traumatic encephalopathy, medical specialists have found.

Former Auckland Blues and New Zealand Maori representative scrum-half Billy Guyton was determined to have been suffering from CTE after his family donated his brain to the University of Auckland's "Brain Bank".

Brain Bank co-director Maurice Curtis said in a statement on Friday that stage two CTE had been noted in 33-year-old Guyton by a New Zealand-based pathologist and seconded by a specialist in Australia.

CTE has been linked to repeated blows to the head in a number of contact sports and is known to cause violent moods, dementia and depression.

Guyton's father, John, told Radio New Zealand those symptoms rang true with his son, who retired prematurely in 2018 after being plagued by bouts of concussion.

"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."

The finding comes as a group of nearly 300 former rugby union players, including England World Cup winners Steve Thompson and Phil Vickery, are taking legal action over brain injuries.

The players allege that World Rugby, the Welsh Rugby Union and the RFU failed to establish reasonable measures to protect their health and safety.

Injuries from head blows are said to have caused other disorders such as motor neurone disease, early onset dementia, epilepsy and Parkinson's disease.

In a statement, New Zealand Rugby said it is taking measures to reduce the danger of head impacts.

"NZR is also supporting world-leading research to better understand the long-term impacts of participation in rugby including a focus on understanding any link between concussion and long-term brain health."