It had been chiselled off from its super-glue backing and there were signs a car had been driven close to the grave to allow the thieves to get in close.
Mr Pope said those who took his dad's medallion were "just mongrels" with no respect for the sacrifice servicemen had made so future generations could live in freedom.
Percy Pope died in January 1974, aged 81, and the bronze medallion featuring Simpson and his donkey carrying a wounded soldier had been glued to the headstone, undisturbed for nearly 40 years until now.
Apart from the men and donkey portrayed on the medallion there is also the year 1915 inscribed, surmounted by St Edward's Crown and sitting on a wreath of fern leaves bearing the word Anzac.
The war veteran had been a proud survivor of the war. He had named his only son Leslie Gordon Pope, the two Christian names being in honour of two close mates killed at Gallipoli in 1915.
His son said like many returned servicemen his father would not often talk of his war experiences but would occasionally open up and had left his war diaries which the family retain.
Percy Pope had gone to war with the Mounted Rifles but switched to become a machine gunner.
The veteran's granddaughter Mary Pope said the best outcome would be for those who took the medallion to return it to any member of the Pope family in Greytown. Anyone who could shed light on the theft should also contact the Pope family.
Second hand dealers or owners of shops selling collectibles should also be aware of the theft, she said, and do the decent thing by reporting any attempts to cash in on the stolen medallion.