Councillor Steve Sims has told Radio Scilly that the lack of rain, and the extended hot and dry period on Scilly "could become a crisis" and have an impact on Scilly that lasts for a generation.
The Council of the Isles of Scilly are urging residents, visitors and businesses to do everything they can to conserve water.
Councillor Sims who is also the Lead Member for Place said: "It could become a crisis because a lot of the water (on St Mary's) is under Porth Hellick. We've got big spring tides in the middle of August. If the water levels too low you can get saline intrusion."
This is where salt water contaminates fresh water and floats on top. Councillor Sims added: "It takes 40 odd years to settle."
Plans and contingencies are in place and these include having water sent over to Scilly from the mainland but Councillor Sims said: "If we had to do that, we would do that on a short-term (basis) but what I'm talking about is long-term."
Other possibilities include expanding and upgrading the desalinated water supply on Scilly but Councillor Sims told Will on The Morning Show that this is complicated and "hugely expensive".
The Council of the Isles of Scilly are asking everybody to check their water supply for any leaks. A single dripping tap can waste over 5,000 litres of water a year. If you’re unsure about a leak, please don’t hesitate to contact the council's water team.
They also have free save-a-flush bags which reduce the amount of water used each time you flush the toilet. These are easy to install and available free of charge.
If you think you have a water leak or require any other information, don’t hesitate to contact the council’s water team on 0300 1234 105 or firstname.lastname@example.org.
The Council of the Isles of Scilly also added: "You should continue to stay hydrated in the hot weather and drink as much water as you need. We would never ask people to restrict the amount of water they drink."
You can now listen to the full interview with Councillor Sims below.