One of the most important wetlands in the Riverina has been saved from potential conversion into cotton or rice farms in what's being claimed as the largest purchase of private land for conservation in Australia's history.

The deal comes as NSW's Department of Primary Industries investigates another big fish kill in the state's rivers.

The Nature Conservancy has raised $55 million in philanthropy to buy the Juanbung and Boyong cattle stations on the confluence of the Murrumbidgee and Lachlan rivers from landholder Tim Roberts-Thomson.

The Great Cumbung Swamp, formerly owned by the Roberts-Thomson family, will be managed by The Nature Conservancy as part of a $55 million land-for-conservation purchase. Credit:Tim Roberts-Thomson

The purchase will protect almost the entire Great Cumbung Swamp, home to 131 bird species and more than 200 plant species.