The water utility firm runs 770 vans and 300 cars.
The contract will see vehicles arrive from the entire Vauxhall Commercial range, mostly short wheelbase Vivaro and Combo models, based on 48-month/ 60,000-mile contracts, with funding supplied by Hitachi Capital.
Southern Water’s fleet manager Kevin Shepherd said: “On a whole life cost basis Vauxhall’s performance is excellent, while the Vivaro especially offers superb fuel economy.
“Our guys love these vehicles and see them as their mobile tool boxes.”
For cars, every GM model except convertibles is available to drivers based on their monthly allowance, although they can trade up if they are prepared to contribute the extra money.