The hosepipe and sprinkler bans remain in place although the company has granted an exception for gardening businesses and their clients. They can water newly laid turf, trees, flowers and shrubs for 28 days after planting.
Southern had expected an increase in water use in Brighton and Hove and other seaside towns over the long bank holiday weekend.
But instead the rainfall kept water use down while helping to restore levels in reservoirs, rivers and aquifers.
The Brighton and Hove area relies on aquifers – underground sources – and Southern said that these “remain below average for this time of year as they recover from two exceptionally dry winters”.
The company added: “The situation continues to be closely monitored as another hot spell is likely to see demand for water increase by a third as people make the most of the sunshine.”
Water quality and strategy manager Meyrick Gough said: “The wet weather helps reservoirs and river levels but importantly it helps keep gardens watered and water butts topped up.
“This, together with the great efforts our customers are making to use water wisely means that demand is lower than we would expect at this time of year, which helps preserve water resources.
“We don’t want to keep restrictions in place any longer than we have to and the recent wet weather and drop in demand means that it may be possible, depending on the weather and demand for water during the coming month, to lift them much sooner than we expected.
“We will keep customers updated on the situation and I thank them for their continued support.”