Riviera Farms says thornapple (Datura stramonium), also known as jimsonweed, became mixed in with its crop.
Almost 200 people have reported symptoms after eating the farm's baby spinach in recent weeks, including a child in Queensland who was admitted to hospital.
Costco, Woolworths, Coles and Aldi have stripped Riviera Farm's products from their shelves and a recall has been issued.
"The investigations have not identified any other potential chemical, herbicide or other type of contaminant," a spokesperson for Riviera Farms said in a statement.
No other produce from the farm has been impacted but nearby crops will be destroyed as a precaution.
Experts from VicHealth were involved in identifying the weed, which is poisonous to animals and people, particularly children.
Earlier this week, peak industry body AUSVEG urged Australians to keep supporting vegetable growers, saying the recall was limited to spinach from a single farm in Victoria.
Australians who consumed the products were told to watch out for symptoms including delirium or confusion, hallucinations, dilated pupils, rapid heartbeat, flushed face, blurred vision and dry mouth and skin.
People who experience any of the symptoms should seek medical attention immediately.
