Melbourne man discovers extensive model train network underneath house

How? It was accessible through a door. Nobody - not the seller, agent, himself or any other prospective buyers, or the building inspector he presumably engaged to check the place over before signing contracts - thought to look behind the door?

How can you buy a house without checking out the foundations/basement yourself or by a pro?

All the home inspectors I looked at (Victoria, where this house is, plus Tasmania) were all quite clear that they would only access areas they could find a way in. Closed up areas, wouldn't be inspected by default.

In fact things like attic hatches are supposed to be sealed ane so even though seen the inspector is not allowed in the attic. (Unless there is other evedence of a problem, though they need to repair the seal in that case.