Council reported the theft, believed to be valued at more than $1000, on Wednesday.
Sergeant Tony Woodbridge, of Oamaru, said water thefts occurred in the district occasionally.
"People can tap into a water supply and take it."
He believed the council was alerted to the theft by a significant drop in water pressure in the surrounding area.Council regulatory services manager Lichelle Guyan would not say how much water was taken or where it was taken from.
"As the police are investigating this we are unable to comment at this time."
However, council assets manager Neil Jorgensen said rural water thefts from council-controlled supplies were an ongoing concern.
Some people believed water was there to be taken, but the water system was set up so everyone on the supply could access what they paid for and thieves were affecting their neighbours further down the line.
