Thieves have filled cars with stolen number plates and left without paying
Motorists have been warned to be vigilant after a series of fuel thefts, including some where petrol tanks were punctured to drain the fuel.
Gwent Police said there were 194 cases of fuel theft in the area between 1 May and 15 July.
Many involved thieves filling up cars with stolen number plates, then leaving the garage without paying.
But the force also warned drivers to be aware of thieves who steal petrol directly from cars.
Some of these cases involved thieves puncturing the fuel tanks and draining the petrol, which Gwent Police described as "an extremely dangerous practice".
To protect fuel against thieves, Gwent Police advised motorists to park cars in garages where possible, and to make sure the vehicle has a lockable filler cap.
They also asked people to report the theft of number plates immediately and to tell police about anyone acting suspiciously around unattended vehicles.
Garage owners are advised to make sure CCTV cameras are able to film faces and number plates at each pump and to report any thefts to police immediately.
The Gwent force said it was using automatic number plate recognition technology to identify offending vehicles and added that it entered all cases of stolen number plates into the police national computer.
Dyfed-Powys Police said there were 40 cases of people leaving petrol stations without paying during the same period.
Last month the RAC said it had experienced a surge in calls from drivers whose fuel had been stolen.