They are appealing for assistance after damage and theft was caused to Derryvolgie House in Lisburn.
A member of NI Water staff carrying out routine checks discovered the gates of the site had been cut open on June 10.
The intruders were still on the premises and were seen stripping materials from the roof and from the inside of the main building.
Other items from the site had already been removed. The PSNI were called to the scene and the intruders were apprehended.
Noel Hamilton from NI Water said, “This isn’t the first time this particular building has been vandalised, there have been several previous incidents. Thanks to the quick reaction of staff, this time the intruders were apprehended by the PSNI and unable to proceed with their further attempt of theft.
“It is thought the intruders intended to remove steel, roof slates and other items of worth from the vacant premises in Lisburn.
“NI Water takes all reasonable steps to ensure the safety of our site, but we would point out that our sites should only be accessed by trained personnel.”
The PSNI can be contacted on 0845 600 8000.