All of its sites are closed with immediate effect.
The closures include Landal Kielder Waterside and anybody with accommodation bookings is urged to contact the holiday provider Hoseasons to discuss rescheduling their break, or alternative arrangements.
All associated public toilets and car parks are also closed.
David Hall, Northumbrian Water's Head of Commercial Strategy and Transformation, said: "In support of the Government's efforts to defeat Covid-19, it is with an extremely heavy heart that we have made the decision to close our leisure sites and reservoirs to the public with immediate effect. We also believe it is important to close our sites so that we can support rural communities who have been overwhelmed in recent days by a significant number of visitors who are not observing social distancing protocols and putting a strain on villages and their resources.
"The closure is currently expected to last until April 27, but this will be under constant review and may be extended.
"People who have fishing permits or caravan park bookings will be refunded, as these are being cancelled in line with the closures, as are all activities booked at Landal Kielder Waterside. This process is now well under way.
"We hope all of our customers, and those who enjoy visiting our sites, will understand that this closure is absolutely necessary, and we urge everyone to follow Government guidelines and support the fight against this pandemic in every way possible."