Puncture potatoes on all sides with a fork. Coat each potato with olive oil then season with salt and pepper. Place each potato on the end of a separate sheet of aluminium foil. Crush thyme and rosemary sprigs by hand and place 2 sprigs of each and 1/4 of the crushed garlic next to each potato. Roll potatoes and herbs in the foil and fold over sides to seal.
Place wrapped potatoes on the preheated BBQ and close lid. Cook 40 minutes turning occasionally until tender.