William was playing on a porch. I have not indicated William was playing on the deck or verandah. This deck / verandah is raised quite a distance above the ground. The stormwater tank which appears to quite a considerable size is situated under the raised deck. The access point is on top of the actual stormwater tank, not from the deck/verandah above it. In the attached it is on the righthand side under the raised deck. He was a tiny framed child so he could have become stuck behind it as well, especially if he squeezed behind it in his hide n seek game. It is right where FM described where he had been playing. Going down towards the Celery Tree.
The porch William had been playing on and was photographed on is in a different location relative to the stormwater tank.