Work awarded includes the engineering, procurement and construction of the Chichester Solar Farm, about 60km of transmission line, building two substations and upgrading another.
The solar farm is part of the Chichester Solar Gas Hybrid project.
The project involves building the solar photovoltaic generation facility at FMG's Chichester Hub iron ore operations and a 60km transmission line linking the Christmas Creek and Cloudbreak mines with Alinta's Newman gas-fired power station and a 35MW battery facility.
Downer CEO Grant Fenn said he expected an efficient integration of the solar farm and supporting power infrastructure into Alinta's existing Pilbara network.
Alina managing director and CEO Jeff Dimery said working together the Alinta and Downer were on the cusp of showing that renewables could drive Australia's economic powerhouses forward.
The project is expected to be completed in the first half of 2021.