The round brass plaque, similar to the one in the photo, was engraved with the name Samuel Corrigal.
It featured Britannia and a lion and was issued to the next-of-kin of all British and Empire service personnel who were killed in the war.
Senior Constable Darren Cox, of Omakau, said it was taken some time after Tuesday.
''The theft of this plaque is a reprehensible act,'' he said.
''Not only is it the theft, but the significance of the plaque to the family that has caused a lot of distress.''
Anyone with information about the theft can contact the Omakau police station on (03) 440-2190 or provide information anonymously on Crimestoppers 0800 555-111.
