Southern Water announced restrictions on customers in Hampshire and the Isle of Wight from Friday, with the measure following exactly one week later for South East Water customers in Kent and Sussex.
Mr Dewhurst also urged people to take precautions in hot weather to avoid overheating, including closing curtains and windows during the day.
“As the heat wave develops, stay out of the midday sun, try to stay in the shade if you are outside and it is best to close the curtains during the day and open the curtains and windows at night to let in the cooler air. inside,” he said.
“Drink plenty of water and make sure you stay hydrated.”
He added: “In general, August is a wetter month, so it’s quite unusual to have prolonged dry weather in August.”