Duke has spoken regularly about climate change issues.
He urged governments on Thursday to do more to protect the environment from companies trying to profit from fossil fuels in Africa.
“To protect the Okavango River Basin, we call on the world to stand in solidarity with us, our allies and local communities in advocating a full moratorium on oil and gas development in the region,” the Duke and Mr Mangundu said.
They added: “Now the choice is simple: either we honor our natural and life-sustaining ecosystems and preserve them for future generations, or we use them on a path to permanent destruction.
“Will you stay with us?”
The delta is one of the largest freshwater wetlands in South Africa, the main source of water for one million people, and one of Africa’s richest places in terms of biodiversity.
Ahead of his 2019 visit to the region, Harry said in an Instagram post from his official account: “This ecosystem, known to the locals as a ‘source of life’, plays an absolutely crucial role for the planet, people and wildlife. This is our only chance to save this magnificent final Heaven.”