"It is going to take some fairly serious funding to get it off the ground," he said.
Other issues included membership of the trust, funding its work, getting the technical information to make decisions and identifying areas for conservation and development.
He did not see the agreement as a win for one sector or the other - it was a collaborative agreement and had to work for everyone in the future.
Minister of Conservation Nick Smith said the challenge and focus for the future was ensuring all parties worked together to implement the agreement.
The agreement represented "a significant change" to the way issues would be resolved, rather than the traditional confrontational approach.
It was "a wonderfully balanced document" that provided a shared vision to the future of the region, he said.
Waitaki Development Board director Mike Neilson described the agreement as "an outstanding achievement".
"I'm excited about the future for the people of the Mackenzie country," he said.
Environmental Defence Society chairman Gary Taylor said the agreement was "unique in New Zealand" by establishing a trust to negotiate joint management agreements with private landowners and high country run-holders to protect significant areas.
Reaching agreement had posed some challenges, getting beyond old enmities and position-stating into constructive dialogue.
"The ball is now in the Government's court to introduce the recommended legislation to create the Mackenzie Country Trust, and make a contribution to funding its work," he said.
Labour Party conservation spokeswoman Ruth Dyson said the agreement was focused on freeing up land for development rather than conservation.
The proposed trust was unnecessary, and its purpose and coverage areas unclear, she said.
The agreement did not identify areas for conservation, and she found that "a major concern".
Forest and Bird said the move to protect 100,000ha was long overdue and "just in time to give future generations of New Zealanders a chance to know a precious and remarkable natural landscape".
Its Canterbury-West Coast field officer Jen Miller said the real test would come during the next phase, when a mechanism was developed to ensure the 100,000 hectares were protected.
"The hard part will be translating what's been acknowledged by all the parties - that the Mackenzie is far too precious to lose - into reality," she said.
Otago University-based ecologist and Forest and Bird conservation ambassador, Sir Alan Mark, said the agreement was a fantastic step forward.
"The Mackenzie is a unique piece of our natural history, which supports an extremely delicate ecosystem. This agreement is a very welcome outcome that I fully support," he said.