The NSW government has reached an $8.9 million deal to permanently protect an area of land twice the size of the Royal National Park in north-western NSW, highlighting the role for private landowners in conservation.

The two adjoining properties, Naree Station and Yantabulla Station, span 31,200 hectares and contain nationally significant wetlands that are among the top 20 sites for waterbirds in Australia.

The wetlands, on traditional Budjiti country, are part of the Paroo River catchment, the last free-flowing river of the Murray Darling Basin.

The landowners, Bush Heritage Australia and South Endeavour Trust, are both charities that bought the land with the intention to conserve it. The deal with the NSW government means the protection is now guaranteed in perpetuity and the Biodiversity Conservation Trust will make annual payments to fund conservation.

Bush Heritage Australia healthy landscape manager Brenda Duffy said the charity bought Naree in 2012, while South Endeavour Trust bought Yantabulla in 2015, and both properties had significant grazing in the past.

The deal meant the landowners had long-term funding for conservation management on the property, which Duffy described as a “diverse, open, remote area with significant ecological and cultural values”.

“Sometimes there’s water and everything comes alive and other times it can be dry for a long time and a very different place,” Duffy said. “Each time I visit it, it’s vastly different.”

The land is a habitat for pink cockatoos, budgies, emus, short-beaked echidnas, red kangaroos, and a variety of frogs and reptiles. Scientists have recorded more than 187 waterbird species, many of which are threatened, at least 15 mammal species, including the vulnerable striped-faced dunnart, sandy inland mouse and little pied bat, and at least 350 native plant species, among 16 vegetation communities.