Safeguarding river life is as important as tackling low reservoir levels, the Environment Agency has said.
Bewl Water reservoir has never been more empty since it was built
Southern Water is applying to the agency for a drought permit to fill a reservoir on the Kent and Sussex border with water from the River Medway.
Agency spokesman Peter Midgley said on Monday: "We want to be sure there is enough water for people to drink.
"Equally importantly, we have got to make sure the wildlife in the Medway is kept happy as well."
Southern Water said the south of England's biggest reservoir had dropped to its lowest level.
Bewl Water on the Kent/East Sussex border is now only 36% full - the lowest since it was built in 1975.
Southern Water said it needs the drought permit to help refill the reservoir over the winter.
Hosepipe bans imposed in the summer remain in place across the South East as the region continues to suffer its worst drought since 1976.