The park is in two distinct sections. The eastern part includes: Stockton Sandspit; Sandy and Smiths islands; the bed of Fullerton Cove; part of the north and southern arms of the Hunter River, Tomago Wetlands, Kooragang, Ash, Hexham and Campbell islands. The western part covers Hexham Swamp.
The park was reserved in 2007 under the Hunter Park Estate (Lower Hunter Region Reservations) Act 2006. This Act also allowed for the revocation of Kooragang and Hexham Swamp nature reserves and the reservation of the lands formerly in these reserves as Hunter Wetlands National Park. Ash, Campbell and Hexham islands were later additions to the park and were previously Crown land managed by the former State Property Authority and leased to the Hunter-Central Rivers Catchment Management Authority for the Kooragang Wetland Rehabilitation Project.
The wetland system within the park is of international significance and is listed under the Ramsar Convention. It is considered to be of exceptional conservation value, containing the second largest area of mangroves in New South Wales and extensive areas of coastal saltmarsh.