The Hunter Valley has been dubbed "one of the worst examples" of agriculture destroying biodiversity.
"The whole floor of the valley is cleared, essentially. There's hardly a remnant of native forest left," University of Newcastle Professor Michael Mahony said.
"Because it was low land, the whole lot was cleared."
Professor Mahony said coal mines cover 5 per cent of the valley floor and agriculture 90 per cent.
Professor Mahony's comments follow the recent release of a United Nations report on biodiversity that's been dubbed the "Paris Agreement for nature".
The report found about 1 million species face extinction worldwide because of human activity.
The rate of destruction was about 100 times faster than the past 10 million years. Biodiversity was now declining faster than at any time in human history.
Professor Mahony said biologists had been "talking about this for 20 years".
"There are Pulitzer Prize-winning books about it. It precedes climate change as a great concern of biologists," he said.
"Humankind has destroyed so much of the planet. It's been measured and mapped, the history well told."
He said the major reason for the loss of biodiversity was the industrial revolution.
The destruction of habitat could be traced to "the moment mankind started to use an internal combustion engine for agriculture and fisheries".
"One bulldozer is 1000 times more effective than a man with a plough," he said.
"From the beginning of trawlers with engines and nets, you can map the destruction of nearly all the major fisheries in the world."
In the Hunter, it was a "good and bad news story".
While the clearing of the valley floor was the bad news, the good news was that the Blue Mountains wilderness area was on the Hunter's southern edge.
"It's called the Wollemi National Park," he said.
"We can say it's a massive wilderness area and national park that we can be proud of. But then, the only reason it's there is because it's so rugged that agriculture couldn't be conducted there.
"It wasn't because we planned to have a massive national park."
The Barrington range lies at the Hunter's northern end. It includes a world heritage area and national park.
"It protects our water supply - Chichester and Lostock dams," he said.
"The Hunter is funny. It sits between these two wonderful pieces of the Great Dividing Range, which still has a lot of natural forest and habitat."
The clearing of the valley, though, has meant a broad loss of biodiversity.
"Fauna like the eastern quoll would have been common right through the Hunter Valley," he said.
"It's extinct on the mainland. It's also rare for people to see the spotted quoll anymore. And there's a swag of small mammals that people don't even think about, like phascogales."
The Williams River, he said, had been home to the "beautifully-named" Booroolong frog.
"We haven't seen one on the Williams River in 40 years. It's just not there anymore," he said.
In the Watagans, in the Hunter's south-west, lives the giant burrowing frog.
"We now know it's restricted to one site in the Watagans. The population could be less than 50," he said.
"The Watagans also has significant populations of native mammals, but they're all declining very rapidly. Yellow-bellied gliders were once very common in the Watagans.
"And there hasn't been an eastern quoll seen in the Watagans in 50 years. The last eastern quoll seen on the mainland was in the Barrington 20 years ago."
The decline of koalas in Port Stephens has been a concern for some time.
"They are an example of the next animal to fall over. In my own lifetime, koalas were not even listed as threatened. We only added koalas to the threatened species list in the last two decades," he said.
"We used to be able to jump in the car and drive to the Chichester range and show visitors a koala with a spotlight."
The question, he said, was "what we do with the animals and plants that remain".
He said the mantras of "think globally, act locally" and "all of us have to be environmentalists" ring true.
"Blaming government is not the answer. We need to look for solutions," he said.
The NSW government was spending $100 million to save species, but a national plan of action was needed, he said.