The RSPB bought 256 hectares of grassland and salt marsh on Canvey Island from William Morrison Supermarkets in May 2006.
It is the last remaining green space on Canvey Island and the RSPB wants to make it a haven for wildlife.
Work includes a nature trail, viewing points and picnic areas.
Once the works are completed, the new reserve is set to open to the public in summer 2010.
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