Managers at Southern Water say they cannot rule out extending a hosepipe ban which came into force for 100,000 homes in north Sussex on Friday.
The ban for 100,000 homes came into force on Friday
Southern Water said levels at its Weir Wood Reservoir, near East Grinstead, are extremely low. Normally at this time of year it is nearly full.
It is monitoring the situation closely but the restrictions could be extended to the rest of Sussex this summer.
Spokesman Meyrick Gough said the firm had no choice but to impose the ban.
Weir Wood relies entirely on rainfall and cannot be filled by pumping extra water from rivers.
The South East of England had only had 60% of its normal rainfall between November and May.
Sutton and East Surrey Water customers were banned from using garden sprinklers and unattended hosepipes in April because of continued low rainfall.
The regional water watchdog is to quiz the water companies about the water shortage at a public meeting next week.
WaterVoice wants to know what the companies will be doing to manage the situation to minimise the impact on customers.
"We want to ensure that customers' interests are paramount," said Richard Sturt, WaterVoice Southern chairman.
"A major concern is whether the industry has the ability to copy in the longer term if extensive development in the South East goes to plan and climate change results in more extreme weather conditions."
The meeting is being held at Hop Farm Country Park, Paddock Wood, Kent on 16 June.