Not sure about this particular case, but the 'wild men of No-Man's-Land' was a persistent legend of the war, cropping up repeatedly in memoirs and post-war fiction. The Smithsonian did an interesting piece on the topic here. One excerpt mentions the Somme, although it's Germans that have gone to ground in this case:

One of the earliest published versions of the “wild deserters” legend appeared in the 1920 memoir The Squadroon by Ardern Arthur Hulme Beaman, a lieutenant colonel in the British cavalry. No other telling of the legend—at least in print—is as horrifying as Beaman’s. Written just two years after the war’s end, Beaman's tale begins in early 1918 on the marshes of the Somme in northern France. This is where some of the bloodiest battles of the war were fought and Beaman is convinced that he has witnessed two dozen or so German prisoners of war vanish into the ground. He wants to send a search party into the maze of abandoned trenches but is advised against it because the area “was peopled with wild men, British, French, Australian, German deserters, who lived there underground, like ghouls among the mouldering dead, and who came out at nights to plunder and to kill. In the night, an officer told him, mingled with the snarling of carrion dogs, they often heard inhuman cries and rifle shots coming from that awful wilderness as though the bestial denizens were fighting among themselves.”

Certainly most absconders aimed for a better life than scratching around on a battlefield. In the majority of my research, it seems that men who deserted would usually gravitate to the cities and towns of France and Belgium. They were hiding out in estaminets and brothels; or working on desperate French farms that had lost all of their menfolk to the war; or hanging around port towns and harbours looking to smuggle themselves on to a boat, hopefully to start a new life elsewhere.

The idea of those Australians on the Somme is interesting, though, and not entirely impossible. By 1917 the Somme front had moved on, leaving behind a swathe of much fought-over devastated land littered with earthworks and equipment. It's possible that desperate men could live a life of sorts hidden amongst all of this. In favour of the idea is the concept at the time of the 'Battle Zone', the forward fighting area (that included the fire trenches back to the reserve lines and support positions) with a rear boundary some way back that was policed by either Brigade units or Military Police units. They were watching out specifically for men attempting to leave the forward lines without permission, and so acted as a 'back-stop' that could conceivably trap a group of deserters in the forward areas when they might prefer to be escaping. The penalty for being caught leaving the line was, potentially, execution by firing squad. Might these Australians have been unwilling to risk trying to pass the police line, and so felt their best option was to stay put?

Against the idea is the question of how they would supply themselves with food, and even more importantly drinking water. Salvage was a hot issue by this point in the war, with every soldier exhorted not to waste equipment of any kind and to make a point of picking up discarded kit, uniforms, metal etc. as they returned from trench duty and deposit it at salvage dumps at the side of roads, to be collected by GS wagon and taken back to depots for sorting. Dedicated rear-echelon salvage teams scoured fought-over areas for anything that could be reused, so it's not all that likely that the Australians would have an endless supply of forgotten supplies to live off. Raiding bases and depots for food would be a risky business indeed.

Afraid I can't add any more, but it's an intriguing story!