The 6.5-year contract, which is effective July 1 with a 10-week transition that has already begun, is worth A$660 million (US$506.2 million).
Downer will be responsible for all site underground mining operations, including site setup, development to the top of the ore body, infrastructure excavation, production setup and ramp up and bedding in steady-state operations at nameplate site capacity.
The selection of Downer comes after OZ reported last month it had received primary approvals, including the mining lease, from the South Australian and Commonwealth governments.
The A$916 million Carrapateena project, located north of Port Augusta, is expected to produce 4.25 million tonnes annually with an estimated mine life of 20 years. Life of mine average annual production should be approximately 65,000 tonnes of copper and 67,000 ounces of gold.