About 4,000 litres - worth about Â£5,000 - were taken from the site north of Carsphairn near to Windy Standard between 4 and 6 January.
It follows a similar incident in which 1,500 litres were taken between 23 December and 4 January.
PC Ian Dunn said a "large vehicle" would have been used in the thefts.
"The site where the fuel was stolen is located almost a 30-minute drive into the forest from the A713 north of Carsphairn," he said.
"We are seeking the help of those who use the A713 road and want to hear from anyone who may have seen anything suspicious on this road between Wednesday and Friday last week.
"Callers can contact police at Castle Douglas through the 101 number."
Thieves have drained diesel worth thousands of pounds from a tank in a remote part of southern Scotland for the second time in a matter of weeks.