Three men have been arrested after police seized more than 1,600 cannabis plants from a warehouse in Sydney's north-west - the largest hydroponic haul in the state's history.

An undercover police officer removes cannabis plants from the warehouse. ( ABC News: Jessica Kidd )

Police raided the property on Victoria Street in Riverstone and found what they said was $6 million worth of the plants growing in a hydroponic set-up.

Three men were arrested at the scene and taken to Quakers Hill police station, where police said they were assisting officers with their inquiries.

Chief Inspector Graeme Wright said police expected to charge the three men - aged 20, 36, 30 - with cultivating a commercial amount of cannabis.

"To the best of my knowledge, this is the largest seizure that has actually occurred with NSW, of plants that are grown in this method," Chief Inspector Wright said.

He said more drugs had been seized grown in other ways, but this was the largest amount found to be grown hydroponically.

Chief Inspector Wright said the warehouse was split up into 28 rooms.

"Each room is at a different stage of growing," Chief Inspector Wright said.

Loading

"Inside they're all set up with lights and a watering system and it's very, very sophisticated. In my opinion this is an expert operation."

He said police have been watching the site since December and had not seen any legal business activity happening in the warehouse.

"I haven't got any evidence of who they were supplying to, but from the scale of the amount of plants that they're cultivating, it would be fairly substantial."

Chief Inspector Wright said this was one of the more sophisticated set-ups that he had seen.

"An investigation with this result will obviously have an impact to the supply of cannabis in this area."

He said no other arrests were expected at this stage, but officers would continue to investigate.

"If anyone has any information, we would like them to come forward."

He said police were dismantling the operation and the cannabis would be taken away and burnt.