The theft from the compound near to Deughside, off the A713 road, Kendoon, Dalry, happened sometime overnight between Thursday October 8 and Friday October 9.
The power washer is a Sprint make, yellow coloured, and is being valued at over £16,000.
PC Jennifer Ross from Castle Douglas said: “This is a big power washer and measures some 3m x 3m, and weighs about one tonne.
“It would have required a number of people and a large vehicle to take away.”
PC Ross added: “We want to hear from anyone who may have been on the the A713 in the area to get in touch if they saw anything suspicious on Thursday or Friday last week.
“Police at Castle Douglas can be contacted through 101.”