Indian police have cracked a criminal syndicate accused of smuggling more than 50 million litres of crude oil inside water tankers from the country’s largest onshore oilfield.

The theft at the Cairn India oilfield in Rajasthan went undetected for nearly six years until police in the region arrested 25 people for involvement in the smuggling network.

Local media reported $7.75 million worth of oil could have been stolen in total from the oilfield.

More than 75 people, many of whom are drivers and contractors working at the oilfield, are still wanted in connection with the heists.

"The company suspected something fishy was happening and complained to us, and during investigations we found this was an organised ring," district police chief Gangandeep Singla told AFP.

Mr Singla said the drivers were authorised to carry water from exploration sites for dumping but some of the tankers were filled with crude instead.

Police said the drivers were disabling their GPS devices to avoid detection, and police have said more than 30 trucks have been seized, with numbers likely to grow.

The oil was sold to two small factory owners nearby, who stockpiled the crude in underground tanks before selling it on to clients across India.

In February, police in the Uttar Pradesh region arrested nearly a dozen people accused of stealing $15.5 million worth of petroleum from a high pressure refinery pipeline.

The gang had purchased land adjacent to the refinery and dug a tunnel to tap the pipeline owned by state-run Hindustan Petroleum.