Storm overflows are used when heavy rain or rising groundwater overwhelms the sewer network.
Southern Water said reducing the amount of rainwater entering sewers was key to long‑term improvement.
It said more than £70m had already been spent on sustainable drainage, sewer upgrades and AI technology, with action now under way at 95 environmentally-sensitive sites.
It added that in Hampshire more than 200 homes in Fareham and Gosport have roof drainage diverted away from the sewer system and major work was taking place in Portsmouth Harbour to reduce storm overflows.
It added that it has been conducting sewer sealing near Andover, large‑scale separation of rainwater from foul sewers in Cosham and Drayton.
Last year the company discovered a leaking sewer under Southsea pier and an illegal wastewater connection from a block of flats in Southampton, both of which have since been fixed.
It said that on the Isle of Wight, upgrades in Cowes have prevented 57 spills, storage at Appley Pumping Station has been quadrupled, and 4,000 water butts, 200 raingardens and six permeable driveways had been installed to keep surface water out of sewers.