Wedge-tailed eagles glide slowly over the swift, skinny streams as they did more than 50 years ago, when botanist Dr Nancy Burbidge launched the "national park for the national capital" campaign, and 100 years ago, when the Cotter River Ordinance was passed.

National Parks and Catchments regional manager Brett McNamara said the decades-long campaign and that early ordinance played a huge role in establishing Namadgi National Park.

Helping to drive the environmental movement in those early years, Fiona MacDonald Brand was at the public meeting in Canberra to form the National Parks Association and begin lobbying.

Later, she climbed Mount Kelly to see over mountains ranges and valleys to Long Plain and Kosciuszko National Park, which convinced the group this beautiful alpine country must become a national park.

More than two decades later, she was alongside Labor minister and environmentalist Tom Uren, who clapped eyes on the nature reserve saying: "Of course it should become a national park."

On Thursday, 30 years since the national park's declaration, Ms MacDonald Brand, the National Parks Association's president Rod Griffiths and Mr McNamara marked the anniversary with a visit to Gudgenby valley, where eagles spread their great wings above trees and bushland regenerated after fires, and felled and rotting feral pines.

"Mother Nature has been looking after the environment for millennia, it's only in the last few years we have come along to protect the environment,'' Mr McNamara said.

"We don't really manage the environment in terms of the natural values, what we do is manage impact and interaction of people on and in the environments.''

That means using a helicopter to get into inaccessible areas to bait wild pigs, heavy machinery to dig out rabbits, rounding up brumbies in the Cotter catchment, and controlling deer, which are spreading across south-east Australia.

“On the positive sides are the bush-walking, mountain bike rides, horse trekking. It really is the people’s park,’’ Mr McNamara said.

“Years ago there was this mentality that national parks were all about locking areas up and putting up gates and putting people out. That is certainly not the way we view and manage the park, it’s about that community involvement and engagement across the spectrum.’’

Mount Tennant and the northern section of the Brindabella Range were added about 1992, making the park 106,000 hectares, almost half the total land of the ACT.

Mr Griffiths said if more was added to the park, he would like to see the lower Cotter included to protect the water supply for the new dam.

"Namadgi comes all the way to the edge, and then there are little pockets of beautiful stuff, but then there’s all these pockets where pine forests have disappeared, but you can see growth coming through," he said.

Land north of the area was regenerating pine wildings quickly.

“Because the ACT forests aren’t actually quite as operational as they used to be, there's no management of those," he said. "It is actually probably a significant fire risk for us.''

Prolific Canberra author and ecologist Jerry Olsen discovered insights into Namadgi's wildlife from studies including helicopter surveys of peregrine falcons on cliff tops, where the weather is harsher and they feed on bigger, potentially more dangerous birds, like currawongs and ravens.

Researching the falcons' diet, Mr Olsen found the remains of a long-tailed jaeger, a sea bird not previously recorded as prey in Australia, which he suspects was blown 100 kilometres inshore.

Mr Olsen has studied birds of prey around the world and, compared to other countries, Australia’s birds of prey are less inclined to breed at high altitudes.

For peregrine falcons, the highest altiutide is 1200 metres at Namadgi, on a par with Scotland.

Despite the remoteness of the landscape, falcons have found plenty of starlings and rock doves to prey on and flocks of domestic racing pigeons.