The utility firm confirmed which firms were awarded contracts under its £3.7bn capital delivery framework covering major infrastructure and works.
The strategic partners will deliver new pipelines, pumping stations and treatment-works upgrades.
Southern Water said period eight of the framework is the largest it has ever undertaken, adding that it also responds to demands from the regulator Ofwat for investment in infrastructure.
Southern Water chief financial officer Stuart Ledger said: “This announcement marks an important milestone in our preparation for the next regulatory investment period, 2025-30, and beyond.
“Southern plans to more than double its investment over the next five years to meet the needs of customers and the environment. I am confident that, working together with our partners, we will fully meet this challenge.”
The framework has a second, ‘low-complexity delivery route’ group that will focus on waste such as supply networks and sewage.
Among those in the first group are Costain and MWH Treatment, which are operating as part of the CMDP+ joint venture.
According to the firms, their award is for two lots with an initial seven-year term – worth at least £500m to Costain – with an option to extend it by up to five years.
Costain has worked with the utility firm during every investment period since 2000.
MWH Treatment chief executive Paul Bresnan said: “We will see the need for continued innovation, along with more sustainable solutions that provide great value for customers with increased resilience against the impacts of climate change and population growth.”
Kier Integrated Services and Galliford Try Construction have been awarded places on the strategic-delivery-partner framework for non-infrastructure, above-ground works.
Morrison Water Services will deliver infrastructure works.
The firms awarded work on the low-complexity delivery route are Clancy Docwra, United Living Infrastructure Services (ULIS) and Barhale. They are contracted for works on the water side.
ULIS has valued its work on clean-water design and build scope at £210m. It will help build potable water bulk supply and transfer arrangements between Southern Water and the neighbouring Sutton and East Surrey Water. The firm will work across Hampshire, the Isle of Wight, Sussex and Kent.
Clancy Docwra, J Browne Construction Co and Morrison Utility Services will be on the wastewater side.
Clancy said its work will include new water mains, sewer rising and gravity mains, infiltration reduction schemes, rehabilitation projects and pump-away schemes. It will also work on waste developer services and infrastructure projects.
The firm said its part of the deal will run for five years, valued at £400m across two lots. There is also the possibility of a three-year extension.
Clancy executive director Ronan Clancy said: “The new regulatory period presents an excellent opportunity for the industry to invest vital resources into expanding and reinforcing the integrity of the country’s pipelines.
“Clancy’s wealth of experience in delivering capital investment programmes, as well as in the maintenance of water and wastewater networks, means we are well-placed to support Southern Water in ensuring the network is resilient and efficient to maintain for the future.”