He very quickly made an impression.

"I located these fossilised bones, mineralised and jumping out of the wall," he recalls from the dining table at his Canberra home.

"It had these big laws and jaw."

The then-15-year-old would later learn he had discovered the fossil of a diprotodon, an extinct relative of wombats and koalas that is the largest known marsupial to have ever lived.

"That spurred on a lifetime desire for discovery," Mr Johnston says.

"Later, that become a yearning to get greater recognition of the value and worth of Australia's Indigenous heritage."

Mr Johnston wasn't always in touch with his Aboriginal roots.

"My mum was white but I had an Aboriginal father and we ended up in the system of the Stolen Generations and adopted out to white families because of the colour of our skin," he says.

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But Mr Johnston is extremely grateful to the family that took in five children, including himself.

He describes his adoptive parents as "champions", and says his adoptive mother helped the kids in their efforts to find their biological relatives.

"I found my sister up in Brisbane some years back," Mr Johnston says.

"I'm tracking down the rest of my mob and hopefully having a reunion later this year. It's a lifelong journey."

Mr Johnston also credits his adoptive parents for helping him find the passion that eventually became a lifelong career: archaeology.

"My dad, in the early 70s, became a lighthouse keeper and I grew up along the Great Barrier Reef, including the Torres Strait, in a number of different lighthouses," he says.

"I did school there by correspondence, played a million games of Monopoly with my brothers, had a few fights and went fishing. That was really all there was to do, so I developed an interest in history and heritage, and exploring the caves and the lighthouses.

"My mum said to me, 'Do you want to be an archaeologist?' I was 11 and I said, 'Oh, what's that?' She said, 'Look it up in the encyclopaedia'. So I dusted off the Encyclopedia Britannica 1958, and I thought, 'That's what I want to do'. And I've never looked back."

The family moved from the Torres Strait to Bathurst and Mr Johnston enrolled to study archaeology at the Australian National University, which at the time had only a handful of Aboriginal students.

He become one of the first Aboriginal people to receive a degree in archaeology when he graduated with honours in 1989.

During his time at the university, he became one of the Tjabal Indigenous Higher Education Centre's foundation students and developed a skill set that would take him across the country.

He estimates that in the past 35 years he has worked on between 250 and 300 Aboriginal heritage projects in more than 70 communities from Cape York, in far north Queensland, to Point Nepean in Victoria.

"You don't know what you're going to do next week," Mr Johnston says.

"People call you up and say, 'Can you give me a hand with this?'

"I fly regularly between communities, doing jobs, and I've always done that.

"I'm a modern-day walkabout man with a platinum Qantas card. My second home is in the Qantas Lounge somewhere.

"But I spend a lot of time out bush as well, which I love."

Mr Johnston is about to start a PhD in archaeology at the Australian National University, but first has to whittle down 15 possible topics to one.

Other recent passions of his include lobbying for a national Aboriginal heritage commission to preserve sites, and connecting farmers with the Aboriginal community to explore the Indigenous history of their land.

Already this year, ceremonies have been held on farms in the Canberra region to mark the declaration of old quarries Mr Johnston found as Aboriginal Places on private property.

Both discoveries were made when friends Mr Johnston had made invited him onto their family's properties to look at the landscape.

"There's such a negative discourse around Indigenous anything, let alone heritage," he says.

"The media and others have said, 'This stops us getting money and it's a negative'. It's not, folks.

"With projects like this, we're bringing the community together and bringing down these social barriers and prejudices, which are minor, when you think about it, compared to the things people actually have in common.

"Farmers and Aboriginal people, if you bring those people together, they all care about the future of that land and they're embracing these friendships."

Where most people judge a place by what has been built there - houses, offices and other buildings - Mr Johnston sees it differently.

"Our land is like a book; an encyclopedia," he says.

"It has different layers and it goes back a long time.

"I'm looking, as an archaeologist, to see what evidence there is of people living there in the past and interacting with the environment. It's everywhere. It's like a footprint."

Despite 35 years working on sites across the country, Mr Johnston's passion for new discoveries has not diminished.

In fact, he says he is more motivated now than ever to spread the word about Australia's rich Aboriginal history.

"When we go out and identify a site, it's exciting," he says.

"You think, 'Oh my God, that might be a bit of sheep poo there, but sitting next to it is a 6000-year-old stone axe'.

"That's the cultural landscape existing in the modern landscape of today.

"A stone tool can be identified as raw material or a blade used for a spear point or whatever it may be. It may be something that was made specifically for an initiation ceremony.

"With the science of archaeology, we can do residue analysis to see what it was used for.

"That's just the tangible and the physical, but with it come the spiritual and cultural stories passed down by the elders.

"As I always say with a significant site: one place, many stories."