Thieves stole a tractor, trailer and gardening tools when they broke into Pensthorpe Nature Reserve, near Fakenham.
The burglars broke in through a padlocked gate at the reserve, about a mile from the A1067, and took the gardening equipment from an outbuilding.
Police want to hear from any witnesses to the burglary, which happened some time between 7.40pm on May 27 and 6.10am the following day.
Anyone with information should contact PC Suzanne Pulfer at Fakenham Police Station on 0845 456 4567 or call Crimestoppers anonymously on 0800 555 111.
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