Flotation circuit additions at Cadia will increase processing capacity from 33 million tonnes per annum to 35Mtpa and improve gold and copper recoveries.
The estimated cost of $175 million is down about $5 million on the estimate a year ago and has an IRR of 21%.
Meanwhile, at Lihir in Papua New Guinea, plant additions and upgrades expected to cost $61 million will slightly improve gold recoveries.
The investment at Lihir has been calculated to have an IRR of 32%.
Shares in Australia's biggest gold miner were up 2% to A$30.90 in morning trade during a positive session for the ASX gold patch.
The stock reached a 12-month high of $38 in August.