Who dug the Japanese tunnels?
Here's a page to gather any references to who dug them. First up is a mention in James Hayes' book, "The Great Difference. Hong Kong's New territories and its People 1898-2004":
Labour requirements for Military Works
The local self-governing offices were expected to respond to Japanese demands for assistance of varied kinds. In Tsuen Wan and Sai Kung, the most notable was for labour to construct access roads to the Japanese wireless station on Tai Mo Shan and from Clear Water Bay into Sai Kung. labour was also required in places like Mui Wo (Lantau) and Lamma Island to make tunnels and caves for coastal observation and defence. Villagers said that conscription was not applied, even though many hands were required and reported gangs of over 100 workers. Men, women and children were glad to come forward, for the sake of the rice that was doled out in lieu of cash payments. "One scoop for us men, and half for women and child workers", as one of the Tsuen Wan villagers recalled. The Sia Kung experience was similar.