The illegal plantation, located 80km north-east of Dubbo was an example of organised crime, senior police said after a dramatic pre-dawn raid yesterday.

They estimate the crop had a street value of $50 million.

More than 200 armed officers, including 40 from the Orana Local Area Command, swooped on an 890-hectare property located between Gilgandra and Mendooran.

Late yesterday up to 10,000 cannabis plants had been pulled from the ground and more, possibly twice that number, is expected to be ripped up today.

Police believe the crop is possibly the biggest found in NSW history and it could take up to a week to find and destroy all the plants.

A firearm was also located on the property on the Mendooran road 30 kilometres north-east of Gilgandra.

A number of Dubbo police officers, including Inspector Bruce Moon, guarded the gate of the farm and directed traffic coming to and from a police command post set up at the property.

The farm gate was clearly visible from the road, while a small, country-style cottage could be seen from the gate. A herd of cows grazed in the front paddock.

Senior local police officers, including Orana commander Ian Lovell and crime manager Detective Michael Willing were at the property for most of the day .

State Crime Command detectives believe the raid has smashed a highly organised and sophisticated drug syndicate.

Drug squad commander Detective Superintendent Paul Jones said the raid was carried out by his detectives and officers from the State Protection Group, airwing unit, dog unit, the western region and Vikings.

"It has probably been about 20 years since we have seen a crop of this size," he said. "This drug operation was very well organised with pumps, tanks and a sophisticated drip-feed watering system.

"It was a complex operation and I would say it's an example of what organised crime looks like."

The crop had been planted in three locations on the property.

Plants were growing among trees which camouflaged the plantation from the air.

Most of the plants were fully mature - between one and two metres in height - and had been in the ground for some months.

They were watered by an elaborate drip system, linked to tanks and dams on the property in a set-up that police estimated could have cost hundreds of thousands of dollars.

Accommodation for workers was also found on the property.

Strike Force Keay had been investigating the criminal enterprise since October last year.

"This police operation came about because of hard-fought intelligence and at this stage there is no link to any other crops found in this area," Detective Superintendent Jones said.

"This was a large crop and when you take players like this out we would be disappointed if it didn't have a significant effect on supply."

It was likely the crop was destined for markets in Sydney and interstate, he said.

Western region commander Morris West praised the involvement of local police in the raid.

"The Orana local area command and the western region contributed significant staffing to this operation," he said.

"Local officers acted on short notice and they did so brilliantly.

"They've achieved a good result and morale among the troops on the ground is high."

Police have charged two Cabramatta men, aged 41 and 37; a 39-year-old Yagoona man; and a 46-year-old Fairfield man with one count each of cultivating a prohibited plant.

A 62-year-old man and a 24-year-old woman, both from Prestons, were charged with one count each of knowingly taking part in the cultivation of a prohibited plant.

A police spokeswoman said five more men were in the process of being charged and one woman was currently being questioned by police in relation to the operation.