Nine heavy-duty vehicle batteries were stolen from plant machinery at a site near Croughton.
It happened between 10am and midnight on Monday, 21 April at a site next to Lake House, just off the B4031.
In addition to the theft, a substantial amount of damage was caused and 500 litres of red diesel was siphoned from various vehicles.
Officers would like to hear from anybody who may have seen suspicious activity at the main gate to Lake House, which is where the offenders are believed to have entered the site.
Witnesses to this incident, or anyone with information, should call Northamptonshire Police on 101 - alternatively, information can be given anonymously to Crimestoppers on 0800 555 111