The five-year agreement includes an option to extend by a further three years into AMP8 from 2025 onwards.
The contract will include Morrison providing automated and visual meter reading and data collection services, carrying out two million visits annually.
The group, part of M Group Services, will manage customer enquiries, appointments, scheduling and exceptions processing of up to 500,000 items each year.
The successful tender was one of six bids to deliver the support and reading services.
Simon Best, managing director of the M Group Services energy retail division, said: “Our expertise in field data collection, meter reading and meter inspections is core to our service offering and we will use our experience and strength in these areas, teamed with strong logistical planning and our unique data intelligence capability to ensure we return high quality outputs and deliver a first-class end experience for Southern Water customers.”