Police say a marijuana crop found in an Adelaide Hills forest would have produced about 33,000 street deals.

About 100 plants were found in isolated and steep terrain three kilometres into the Mount Crawford Forest.

A complex watering system was used to irrigate the crop.

Detective Inspector Peter Giles says a man was arrested at the scene.

"Police located this cannabis crop at Mount Crawford forestry reserve and arrested a 46-year-old man from Salisbury who was allegedly carrying out some of the cannabis from the crop site," he said.

It is alleged the man was pumping water from a creek into rainwater tanks to irrigate the plants.

A tent and about 100 bags of potting mix were also found.

Detective Inspector Giles says it would have taken considerable effort to set up the complex operation.

"As you've seen it's extremely isolated and would have been very difficult bringing in some of the elaborate equipment such as the tanks that we've seen on site," he said.

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