GREAT Southern police have discovered a massive cannabis plantation north of Walpole, seizing about a $1 million worth of the drug.

After receiving a tip-off about the hidden plantation, police yesterday found 420 plants, some as tall as 4m, in bush land north of Walpole, about 400km south of Perth.

Police estimate the plants have a street value close to $1 million.

Nobody has been charged and an investigation is continuing.