Police believe fuel price rises have prompted thieves to target tanks at farms and goods yards across rural Dumfries and Galloway.
Rural areas have been hit by diesel thieves in recent weeks
In the latest incidents diesel was stolen from two sites in Kirkpatrick Fleming between 12 and 15 May.
A spokesman for the police said a cheap padlock could stop the theft of thousands of pounds worth of fuel.
The force has appealed for witnesses to the recent incidents and has also offered security advice.
Diesel was stolen from a building site at Elmside between 1630 BST on 12 May and 0830 BST on 15 May.
In the second incident diesel was stolen from Kirkpatrick Fleming church at around 2245 BST on 14 May.
FUEL SECURITY ADVICE
Tanks should be sited in clearly visible places
Control switches must be in secure buildings
Locking devices should be on all tanks
Install extra security like lighting or CCTV
A white Mercedes was seen in the area at the time of the incident.
Police said they were keen to trace the occupants of the car, who were described as two men in dark clothing.
A spokesman for the force said that the crime appeared to be growing more financially attractive for thieves.
"The recent price rises have encouraged thieves to target fuel tanks at farms, goods yards and other places of business across the region," he said.
"As some of these tanks can store thousands of pounds worth of fuel it is worth considering a £2 padlock to protect them."
The force said local crime prevention officers would be available to give additional security advice.