Mercury agreed a fee of NZ$143.9 million (US$100 million) or NZ$2.30 (US$1.58) per share to acquire 19.99% in Tilt from utility Tauranga Energy Consumer Trust (TECT).
It also has an option to buy TECT’s remaining 6.8% share in Tilt in the next six months. Tilt's major shareholder is New Zealand-based infrastucture invesment firm Infratil with a 51% share.
TECT is a minority shareholder of Trustpower, which in turn formerly owned Tilt Renewables until it split in October 2016.
Tilt Renewables has a wind and solar portfolio of nearly 2.4GW, either in operation of in development in New Zealand and Australia.
It has seven wind projects in operation, with an eighth due online in July.
Mercury chief executive Fraser Whineray said: "Tilt has established a robust portfolio of operating wind farms in both Australia and New Zealand, as well as consented renewable generation projects which it can bring to market when conditions are right.
"The investment will allow Mercury to meaningfully participate in Australia’s accelerating transition to renewable energy sources."
In its 2018 financial results, Tilt reported a 9% fall in group revenue to A$158 million ($118 million) and a net loss for the year, after tax, of A$2.8 million ($2.1 million). Tilt is listed on the Australian and New Zealand stock exchanges.
The result was in part due to lower generation totals in both Australia and New Zealand.
As well as providing power to its 400,000 customers, Mercury sources low-carbon electricity from its own hydro and geothermal generators and provides small-scale rooftop solar solutions.