A witness quoted in the report said McMahon passed his car "like I was standing still. He was flying. He was going like hell".
Mr Austin does not believe it would have been the first time McMahon had done similar speeds on his bike.
"You don't risk someone else's child," he said.
"She trusted people, she wouldn't have seen the danger.
He said as an adult and parent "we have a responsibility to protect and keep safe our children and their friends when they visit, not doing so is unacceptable".
Coroner Christopher Devonport issued his findings yesterday, stating McMahon had been estimated as travelling at speeds between 135km/h and 140km/h and "in excess of four times and close to five times the legal limit" of alcohol in his blood.
Toxicology results indicated McMahon was using morphine, tramadol and other medications which, combined with alcohol, would have contributed to the crash.
Toxicologist Samantha Coward estimated McMahon would have consumed between 22 and 42 standard drinks prior to riding the motorcycle.
Use of his medications with alcohol would potentially have affected the central nervous system depressant effects of the drugs, possibly causing drowsiness.
McMahon's GP, Dr Tony Jackson, said McMahon had been prescribed drugs for anxiety and depression since 2004, and for alcoholism since 2005.
He was also on medication for chronic pain relief for an ankle injury, which had reduced his mobility to the point of needing crutches in January 2015.
After the crash a friend of McMahon told media the ride had been a Sunday morning "leisure ride" because Natalia liked the bike.
She was the girlfriend of McMahon's partner's son.
She had finished Kuranui College in 2014 and been working as a delicatessen assistant at Pain and Kershaw in Martinborough.
Mr Austin described her in his partner's words -- "she was the high note in the song of our day".
"Talia was a beautiful and talented girl with a fantastic future ahead of her, losing her under these circumstances has impacted on the family and close friends she was loved by, and caused us all more grief and sadness," he said.
"Still, we will continue to remember the sparkle in her eye and her wonderful smile and focus on the fantastic times we all shared and the lasting memories she gave us."
Mr Austin said Natalia had been working towards saving for a future OE trip.
"She had just received notification that she had been accepted for university.
"She hadn't decided on any avenue, but she was very much into her writing.
'She could light up a room simply by smiling'
"But she had got the bug [for travel] like her dad, and she was working to earn some money, and just enjoying being 18.
"She was a Wairarapa girl, proud of Kuranui College and a great advocate for the quality that comes from that college, she could light up a room simply by smiling."
He said the family struggle every day with the loss.
"We're very supportive of each other, we're continually checking on each other.
"We miss her every moment of every day and will do so forever, we are a supportive family and continuously reach out to each other whether it is family, her friends, our friends as we work out how to live life again without her.
"I will focus on her memory, her lovely big personality."
His message to teenagers is, "if it doesn't feel right, don't do it".
