Paranoid cannabis growers dumped a homemade hothouse beside a North Canterbury road.

The 2m x 2m plywood growing unit, complete with 20 mature plants, was found on Kings Rd, just off State Highway 1, at Omihi.

Constable Craig Newman, of Amberley Police, said the occupants of a light rental truck were seen unloading the heavy unit and residents tipped off police.

"I think somebody has got nervous that police were about to execute a search warrant and decided to get rid of any evidence," he said.

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Constable Newman said it was not unusual for police to find abandoned hydroponic cannabis systems "but not with mature plants due to be harvested".

"You would expect they would have taken the plants. Obviously it was all done in a hurry. Someone's got paranoid."

Lined with tinfoil and pink Batts, the box housed healthy 1m plants growing in soil-filled fish bins with lamps, a watering system and timers.

Constable Newman said the set-up was undergoing a forensic examination.

"We're confident we'll find fingerprints or DNA."