At the height of the cannabis-growing season, Oamaru police are asking people to keep an eye out for suspicious behaviour.
The premium growing season for cannabis is between November and March. Oamaru police Community Constable Bruce Dow said at the peak of the season, plants would require watering.
"We ask the public to be aware of activities which can signify a cannabis-growing operation, either small or large," he said.
Mr Dow asked anyone who saw people around odd areas of river banks to pick up the telephone and report the sightings to police.
Rural areas were a favoured growing location for the drug, with plants often grown in remote areas of farmland, near infrequently used sheds, in forestry blocks or alongside streams.
Strange vehicles coming and going from a rural location could also be a giveaway for a growing operation, Mr Dow said.
Contact the Oamaru police station on 03 433 1400.