Sounds like the council or local utilities have an easement for the underground drain. An easement is a limited right for someone else, in this case to have and maintain a drain, within your land. There is also an obligation on the landholder (you or your owners corporation) to avoid doing anything that interferes with the rights of the holder of the easement rights. So, for example, you might be welcome to have a tree there but you might also be obliged to remove it if it ever got in the way of making a repair to the pipe. Similarly, you might not be allowed to build a structure in a location such that its footings would put a load on the pipe but you would be allowed to build outside the easement with sufficiently deep footings that their loading passes below the pipe.
Where I am, we were able to build a wall that run up to the edge of an easement perpendicular to the line of the easement because it would not fall in if the easement needed to be excavated to access the pipes. However, we could not build a wall at 90 degrees, along the edge of the easement, because that could topple in if the easement were ever excavated.