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We are bouncing over a rough, steep gravel road on the hunt for macadamia trees in the wild. Here in the tangled rainforest at Mary’s Creek in Queensland is the macadamia’s main ancestral home.

Ian McConachie has always been partial to a macadamia nut. As a boy in Brisbane in the 1940s he used to climb fences to steal them from people’s backyards. “We’d put them in a little hole in the concrete under the house and hit them with a hammer.”

Now 90, he has spent much of his life devoted to the nut, including developing a tree that has raised $2m in royalties for the Macadamia Conservation Trust, which seeks to preserve this scrappy rainforest plant.

Today macadamias are a $2bn global industry, grown in industrial quantities in subtropical climates in Hawaii, the Americas, Africa and Asia. But there was a time when Aboriginal people traded them for rum and tobacco with the white settlers here on this country.

“It is the world’s finest nut,” McConachie says. “It is an Australian icon and our only native plant that has become a commercial food.”

A couple of kilometres from here lives Macadamia integrifolia, which genetic testing has shown is the genesis tree of 70% of macadamia trees grown commercially around the world.

There are three other species: M. tetraphylla is also grown commercially, while M. ternifolia and M. jansenii are almost inedible, wild and rare.

Macadamias have been cultivated commercially since the 1920s, starting in Hawaii. Australia didn’t start commercial production until the 1960s. Only two to three generations out of the wild is “terribly young in the world of horticulture”, McConachie says. “So we simply don’t know what the potential of the wild macadamia tree is.”

After graduating from industrial chemistry and business management with a focus on food, McConachie spent 35 years in businesses involving macadamia nuts. Later, he developed 20 macadamia orchards in Gympie and Bundaberg for investor-owners. Australia was behind Hawaii in growing commercial crops, with the first domestic processing plant only opening in 1954.

Growers were given a tax deduction to help establish a local industry, McConachie says. This also attracted organised crime, seeking to launder money. “I’ve got wonderful stories which can never be printed. A guy working for us was charged with murder.”

All four macadamia species are native to a tiny patch of Australian subtropical rainforest in south-east Queensland and northern NSW. “Which is pretty much where everyone wants to live,” says Denise Bond, executive director of Wild Macadamia Conservation. “They [the trees] like expensive real estate, which makes them incredibly endangered because we’ve got so little rainforest left.”

Wild trees may hold the key to the next generation of climate- and disease-resilient nuts. McConachie says for commercial macadamia farming, “one of the very few things Australia has got going for it is the wild trees and their genetic potential”.

But more than 80% of the habitat that these trees evolved in over 120m years is reduced to remnant now.

One of the macadamia species, M. jansenii, is seriously endangered, so rare “a bushfire could totally wipe them out”, McConachie says. Their locations are kept secret to protect them. While the nuts are too bitter to eat, “it could be a major tool in terms of climate change because it is so hardy”. The other three species are listed as threatened or vulnerable.

McConachie has spent decades walking the remnant rainforests not far from where he lives, to find and document wild macadamias in situ. In August, he and fellow macadamia conservationist Russell Bennet take me on one of their hunts.

Bennet, a Kabi Kabi man, custodian and elder, who is driving, is known for his ability to spot them. “It’s some genetic ability handed down through the ancestors, I reckon,” he says.

First Nations Australians, though, have received vanishingly little financial benefit from the nut’s global popularity.

When he finds a new undocumented tree, Bennet says, “it’s almost like finding gold. You can go through a lot of rainforest and not see one.” Except in September, when they flower and put on a show. The rest of the year, their dark leaves retreat into the rainforest, easily overlooked.

“They’re quite cryptic,” says Bond, with a couple of trees scattered here and there.

Their modesty, says Bennet, “is their problem – they get dozed over”.

McConachie is “shocked” by the degradation of the trees we find. An invasive creeper called cat’s claw has been smothering them; some have been smashed by storms.

We come to a sudden halt at a “Keep Out” sign. Behind it is logging machinery. The hills are denuded, the small macadamia trees struggling in the dust. “The rainforest is tired,” McConachie says. “Whether the remaining trees are still there is unknown.”

Cultivated trees have significantly less genetic diversity than their wild counterparts, making those that remain “a genetic bank,” says Bond. “If anything were to happen, like a root rot fungus go through the commercial population, or any sort of pest or disease that commercial nuts can’t survive, if all the wild trees are gone you can’t go back and say, lets breed some resistance into the crop.”

They should be safe in the state forests of Mary’s Creek, Amamoor and Imbil, the largest continuous area of habitat – “but they’re not,” says McConachie.

“We can conserve DNA, but we can’t regenerate it. So one of the goals in the future will be to find out if we can store the genetics of the wild macadamias, so that they’re available for breeding for conservation.”

In 1981, McConachie and his wife, Jan, bought a rundown dairy farm near Wolvi. He flirted with other trees, mangoes, lychees, stone fruit, but quickly came back to his great love. After an extensive selection trial, he found a particularly hardy and accommodating tree, named it MCTI and secured planter breeding rights. “It has sold about a million and a half trees,” he says, “and raised about $2m in royalties.” All of that has been gifted to Wild Macadamia Conservation.

“We’re probably all a bit eccentric,” he says of the group, “but we’re passionate about the need to conserve the wild genetics.”