Vandals have damaged fire hydrants in Draperstown and Magherafelt areas, according to NI Water.
A hose pipe ban came into force last Friday as the heatwave continues.
Since Friday the company has dealt with 18 incidents, including those in the two south Derry towns.
It appealed to the general public to help them stop the vandalism.
“It needs to stop before the unthinkable scenario occurs where a fire breaks out and there is no water for the NI Fire and Rescue Service to deal with it,” said a spokesperson. The company said the vandalism was not “harmless fun”.
“The reality is, that as they play in the water, homes and businesses are suffering low water pressure or no water at all”.
NI Water has said that demand for water is 25% higher than usual which is down from 30% over the weekend. As a result of the increased demand and sunny weather, a hosepipe ban was imposed at 6pm on Friday. The last hosepipe ban in Northern Ireland was 23 years ago in 1995. The ban applies to domestic usage only.
“We do not want the ban to continue any longer than is necessary, however, the ban will not be lifted until we are confident there is enough treated water to meet the required water demands,” NI Water’s chief executive officer, Sara Venning, added.
“Demand remains high, particularly during peak times, so we are still urging people to conserve water. We continue to maximise our water production and I am grateful to our dedicated workforce who are working continuously day and night to ensure we keep customers in supply.” Anyone who has witnessed vandalism should contact the PSNI or its Waterline on 03457 440088.