The company will design, supply, fabricate, transport, construct and commission the non-process infrastructure required to support the expansion at the project.

Design work is due to begin immediately.

"This contract highlights the strength of our experience and shows how far we have come from providing construction contracting in the Pilbara in 1979 to offering design, fabrication and construction in 2014," Decmil CEO and managing director Scott Criddle said.

"Rio Tinto is a valued client of Decmil and the award of this contract strengthens our relationship and further demonstrates our expertise within the iron ore sector."

The 6 million tonne per annum expansion of West Angelas to 35Mtpa is part of Rio's broader Pilbara iron ore expansion to 290Mtpa and eventually 360Mtpa.

West Angelas sits between Newman and Tom Price.

Shares in Decmil remained at $2.01, while Rio last traded at $60.60.