Detectives on Wednesday raided a 50-hectare property at Gibberagee, southeast of Lismore, and allegedly seized 7200 cannabis plants and about 50 kilograms of cannabis head with a potential street value of $21.6 million.

The property, allegedly used solely for the purpose of marijuana cultivation, contained 20 industrial grow houses, each equivalent to the size of an Olympic swimming pool.

Kien Sy Ngo, 20, Khac Ngoc Mai, 34, Trong Tung Tran, 35, and Giang Hoang, 37, were arrested at the property and charged with cultivating large commercial quantities of a prohibited plant and participating in criminal activity.

NSW Police Assistant Commissioner Stuart Smith said the investigation had dismantled a syndicate operating across the state.

He said 12 people had been charged as part of investigations into the syndicate and $41 million worth of cannabis seized.

"Criminals who profit from cultivating these drugs are often known to reinvest funds into other criminal enterprises that directly impact the communities of NSW," Mr Smith said in a statement on Thursday.

Ngo, Tran, Mai and Hoang all faced magistrate Jeffrey Linden at Lismore Local Court on Thursday. They did not apply for bail and it was formally refused.

The four men will next face court on September 16.

Australian Associated Press