Council said the information it provides is critical during significant weather events to help assess river behaviours, flood risk and potential impacts on the community.
Regional council chairwoman Sophie Siers said the theft was “unbelievable and completely unacceptable”.
“Stealing essential monitoring equipment during a cyclone directly undermines our ability to keep communities safe.
“After what Hawke’s Bay went through during Cyclone Gabrielle, anyone involved in this should be ashamed.
“This isn’t just theft, it’s putting lives at risk.”
She said replacing the system will be expensive, and the council is now assessing options to restore monitoring for that part of the river.
Anyone with information is asked to contact police and quote the file number 260415/0987.