A finding by Coroner Katharine Greig has ruled Smith's death was self-inflicted.
The 30-page decision outlines a history of serious head injuries Smith suffered during his motorsport career and the likelihood that ongoing effects from those injuries "played a role" in his death.
The decision also warns that a traumatic brain injury sufferer is more likely to experience a major depressive episode and up to four times more likely to commit suicide.
Smith, a carpet layer, had been married for about six years to Carrie-Jane Smith. They had a 3-year-old son together and his wife was pregnant with their second child. The couple had recently separated.
The decision says Smith had suffered serious concussion and physical injuries during various crashes over the years, including a "spectacular" speedway smash just three months before he died.
In 2009 Smith suffered possible concussion after coming off his bike while racing.
He began suffering headaches after the crash and his family noticed behavioural changes, including irritability.
A year later he fell off a mountain bike and lost consciousness, later experiencing poor short-term memory, mood swings and cognitive issues.
Smith's motorcycle licence was suspended for two years as a result of the injuries.
He and his father Bernard then built a race car and Smith began drift racing, reaching pro level as a rookie. He took up flat track racing but had "numerous crashes" while learning the ropes.
In October 2018 Smith bought a TA midget and began speedway racing at Western Springs.
Smith had no prior history of mental health issues but his wife began noticing significant behavioural changes from Christmas 2018. He became abrupt with their son and seemed low in mood and less engaged. He was also emotional and would often cry.
In late January 2019, Smith rolled his car while speedway racing.
"His father described it as a horrific accident which changed his son's outlook on everything dramatically," the decision says.
"[His mother Jo] Steele said that the car tumbled end to end and that she had seen her son the morning after the crash and he 'looked absolutely dreadful'."
Smith was assessed by an ambulance officer after the crash but did not see a doctor.
Days later on February 2 he came off a dirt bike. He told his mother he was "in a dark place" and agreed to see a doctor.
His mother said there was a noticeable downward slide in Smith's ability to function normally, with memory loss, mood swings, concentration and confusion issues. He reported occasional thoughts of suicide and was put on antidepressants.
Smith rolled his car again while speedway racing in late February and collapsed in his bathroom a week later, hitting his head on the tiled floor. His wife found him "twitching" on the floor with his eyes rolled back. He was taken to Middlemore Hospital for a CT scan.
Smith was confused, disorientated and unsteady. A doctor noted the two recent speedway crashes and advised that Smith may be suffering post-concussion syndrome, referring him to the ACC Concussion Service.
On March 31 Smith's wife left their home after an argument, taking their son. Smith was extremely upset and attempted suicide the following day, later saying he had felt "overwhelmed".
He began seeing a psychiatric nurse informally, who told the Coroner Smith was concerned about the outcome of a head injury assessment and implications for "potential lifestyle constraint".
Smith told an occupational therapist he was struggling with the effects of his most recent concussion "along with the significant stressors in his personal life".
He was seen later that day by a Concussion Service psychologist who concluded that despite his ongoing safety risk, Smith appeared to have a positive attitude towards his recovery and acute mental health service intervention was not warranted.
The following afternoon he told a neurologist he had ongoing vertigo, difficulties judging speed and distance, and did not feel comfortable driving. He was also having trouble concentrating and sleeping.
During the consultation the term "brain damage" was used, which left Smith devastated. He last used his phone at 4.47pm to message his mother. She was concerned he felt "really defeated".
The next morning a concerned family member who could not reach Smith visited his house. He was found dead inside.
Neuropsychiatrist Dr Gil Newburn prepared a report reviewing Smith's clinical history and commenting on the relevance of recurrent brain injuries in suicide.
Newburn said depression and suicide had a "known association" with brain injury and that major depressive disorder was a common consequence of traumatic brain injury.
He also cited research showing the risk of death by suicide in traumatic brain injury victims was three to four times that of the general population.
Newburn found it likely that the 2019 concussions played a material role in Smith's suicide.
Newburn said traumatic brain injuries created "stressors" that impacted an individual's ability to cope "and is associated with increasing levels of impotence in their capacity to manage themselves, their environments and their future".
The Coroner said Smith had achieved highly in motorsport but had been in a number of heavy impact crashes over the years and suffered injuries to the head.
"It also appears that for the most part Mr Smith 'dusted himself off' and picked up competing again after crashes – even though at times he exhibited symptoms that with hindsight suggest concussion."
Many of the incidents had occurred at a time when the effects of concussion were less well researched and widely appreciated than today, the Coroner said.
Smith's father said that since his son's death "his mum and I have learned a lot about the brain and know that with the knowledge we know now we would have approached things differently".
Smith's mother said the family firmly believed Smith was a casualty of head injury. She also highlighted the growing awareness about concussion, saying New Zealand rugby was benefiting from a global awareness around concussion in sport.
Smith's family have raised awareness and funding for head injury assessment within motorcycle racing in New Zealand since his death.
The Coroner noted that Speedway New Zealand now had an explicit concussion process.
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