Chinese authorities have disrupted two major underground banking networks that helped transfer money from the mainland to casinos in Macau.

On Friday, Chinese state media reported that police had broken up two major underground banks in Shaoguan in northern Guangdong province in a series of raids on November 9.

The banks reportedly handled over RMB 20b (US $3.1b) worth of transactions, helping over 10k clients across 20 provinces to transfer money on or off the mainland or to exchange yuan for foreign currency.

Police detained seven individuals following an investigation that began in July. Police were acting on a tip regarding a long dormant account that suddenly flared back to life last year, resulting in a flurry of 121 transactions with a total value of nearly RMB 99m ($15m).