The firm has already secured all three major tunnel contracts for the £3.2bn project to deliver excavated potash mineral underground to the coast.
The latest contract includes the fit-out of the MTS conveyor, the maintenance railway, electrical and communications infrastructure, and all other services in the tunnel essential to the operation of the MTS.
In total, the tunnel drive and fit-out is expected to cost around £1.1bn.
More than half of the MTS fit-out price is on a fixed rates or lump sum basis.
The remainder of the MTS fit-out price is based upon estimated prices that will be converted into fixed prices prior to completion of stage 2 financing.
Chris Fraser, managing director and CEO of Sirius, said: “Procurement of the mineral transport system fit-out marks the completion of our major construction procurement programme to support our stage 2 senior debt financing process and is a significant milestone for the company.
“Our efforts are now focussed on the successful execution of our financing plan to fully finance the construction of our world-class, long-life polyhalite project.“