New Zealand Windfarms Ltd is to buy the remaining 50% stake in Te Rere Hau wind farm for $20.1 million.
A unit from Australian investment firm Babcock and Brown is selling the asset as part of a move to reduce debt.
New Zealand Windfarms chairman Derek Walker said the purchase will help the company make money more quickly, as it will get all the revenue from the working windfarm.
The Te Rere Hau wind farm near Palmerston North has 11 operating turbines with another 24 due to be up and running by early 2009. It will have 97 turbines when fully operational.
New Zealand Windfarms will need shareholder approval for the purchase.