The South Island is in the midst of its most severe cannabis shortage in 15 years, a cannabis expert and a Waimate grower say.

The Waimate District cannabis cultivator, who refused to be named, said yesterday the shortage was "a nightmare for consumers" who could not source cannabis at any price.

Abe Gray, curator of Whakamana: the Cannabis Museum of Aotearoa, said the "acute" shortage of the illegal drug meant there was "not a single tinny of cannabis for sale" in the entire South Island and supplies in Auckland were also affected.

Gray said he was not sure what had caused the shortage, but "significant" police operations had taken several tonnes out of the market.

The grower said his own crops were "doing OK" and resisting the drought, despite being stunted by a lack of rain in December. Cannabis plants had deep roots and resisted drought better than grass.

"Cannabis is surprisingly hearty and its roots reach deep in the soil."

The grower said although some local cannabis farmers dug dams to irrigate their crops, he believed such methods caused too much damage to the environment.

He planted his crops in areas which stayed moist in dry periods and surrounded them with peat moss to make the soil "fluffier and more spongy".

Dry weather before harvesting in April and May was important to prevent mould developing in the plants, which trapped water around their buds easily, the grower said.

"Ideally we only want it to rain once a week or so through April."

He believed Timaru and Ashburton's populations were too low to sustain large numbers of indoor growing operations.

Aotearoa Legalise Cannabis Party leader Julian Crawford expected the shortage was likely to ease at harvesting time.

The grower said varieties which grew further north would soon be ready for harvest but "whether any of that will make it our way or get soaked up by the huge demand all over the rest of the country remains to be seen".

Timaru police Detective Senior Sergeant Marion Neill said police assessed all information they received about drug offending. Police could make inquiries and execute search warrants if they had sufficient information.

Police were always interested in information about drug-related offending, including the cultivation of cannabis.

Neill encouraged people with information about cannabis cultivation, including "unusual activity in rural areas" to call police or the anonymous Crimestoppers hotline.