More than £1,500 worth of copper cabling was stolen from the sewage treatment works on Sloop Lane, Scaynes Hill, according to Sussex Police.
It is believed the offenders came across the fields at the rear of the premises where the fencing had been cut.
Anyone with any information can call Sussex Police quoting serial number 0426 of 9/12/13.
Also, between December 6 and 9 a generator cable was stolen from a Southern Water site on Park Road, Handcross. The serial number is 0099 of 10/12/13 for anyone who can help.
The Sussex Police number for non emergencies is 101.