"She was very upset and called Matt's mother to express her condolences, at which point Matt's mother said, 'What are you talking about?'" Macdonald told news.com.au.
"That was how they learnt of his death. And that began a very long process for them in establishing the facts surrounding how he died."
A preliminary finding handed down by a New Zealand coronial inquest into the young Kiwi's 2015 death was inconclusive. But it pointed to possible nicotine poisoning, likely from the strong tobacco tea Matthew drank as part of the ceremony.
Macdonald said he'd never heard of ayahuasca before he began investigating the murky circumstances surrounding Matthew's death. His quest to find those responsible led him deep inside the jungle of Peru, where he discovered a booming ayahuasca industry.
"People die sometimes. S*** happens"
Many travellers seek ayahuasca for emotional or physical healing. Some crave spiritual awakening, while others are simply keen for adventure. The drink is brewed from local vines and shrubs and administered by a shaman in rituals that date back centuries. The high lasts hours.
"When I started working on this story I expected ayahuasca to be this sort of fringe thing, but when you go to Peru you realise what a booming industry ayachuasca tourism is," Macdonald said.
"It's had a lot of celebrity endorsements, and it's gone from this very traditional, niche thing restricted to the Amazon to something that's nowadays, I'd say, entering the mainstream drug culture. It's now available in Australia; there are versions that are brewed here.
"Beyond that, more and more people are going to Peru to try it."
It's estimated tens of thousands of tourists head to Peru to try ayahuasca each year, and the numbers are growing.
Interest in auahuasca is so strong in regions such the Peruvian port city of Iquitos, it's boom time for local operators. Experts have warned of "fake shaman" capitalising on the trend, while Macdonald said it's also given rise to "gringo operators" - Westerners who organise stays at ayahuasca retreats.
It's a lucrative business. Some tour operators charge as much as $3000 per person for a week's stay at a retreat, often hosting as many as 10 tourists at a time. The shaman earns a tiny wage, and as fasting is part of the experience, costs are low and the profits margins are very high.
Matthew's stay at a retreat near Iquitos was organised by a tour operator, who was not there when Matthew died.
"We tracked him down and confronted him and asked him about his role ... and he accepted no blame for what happened," Macdonald said.
"He wasn't there when Matthew died and he said he was just the middle man, but the reality is that he was the one who took the money. He said to us, and he used the phrase: 'People die sometimes. S*** happens'."
Macdonald's search for answers also led him to the shaman who oversaw Matthew's ceremony, as well as an American tourist who was also there and still struggles with the tragedy.
A firefighter who sought ayahuasca to heal post-traumatic stress, said he had tried to help Matthew as his condition worsened but was told not to worry. He eventually rode with Matthew to town on the back of a bike in the desperate search for a hospital, but it was already too late.
"Your life is in their hands"
The Department of Foreign Affairs and Trade says while it's not illegal to consume ayahuasca in places such as Peru and Ecuador, there is no way to vet tour operators, and there are health and safety risks.
An expert who spoke to news.com.au in October said while many participants had positive experiences with ayahuasca, there were reports of accidental deaths, injuries and sexual assaults involving intoxicated people.
Since Matthew died there have been five deaths in Peru linked to ayahuasca.
"I think what was very stark when we went into the Amazon is these retreats are a very long way from anywhere," Macdonald said.
"You're not on the edge of the town or near a hospital, you might not have phone reception. If something goes wrong, your life is in the hands of the shaman and the people who run the retreat.
"And there's a very different cultural interpretation of the shaman as a natural healer, who believes in the power of natural plant medicines. And if something's wrong with you, they won't think you need a hospital.
"What became very evident to us was that if you do go there and inclined to try ayahuasca, you're putting an enormous amount of faith in these people, and their beliefs and knowledge about medicine and health and emergency care might be very different to what our expectations are."
The report is the first time time Matthew's parents have spoken publicly about their son's devastating death and Macdonald said they were "very brave and lovely", and on the subject of ayahuasca, realistic.
"Matt's mum says she doesn't want to tell anyone how to live their lives, but to be aware, do your research, and know what you're getting yourself into," he said.
"When Matt died there were so many unanswered questions about his death. His family have been fighting very hard to those answers and haven't been able to.
"It was very clear to me there was a story to be told in going to find the shaman and the tour operator, and putting some of those questions to them. One family lost a really bright, happy, healthy young guy, and they don't want that to happen to anyone else."
