Property Boundary Encroachment

I purchased a sectional title property in 2002 and requested the building plans and the surveyors certificate indicating the boundaries. The estate agent responsible for the sale supp**** me with the approved docs and all seemed fine. Some years later 2 permanent structures were erected by myself within the municipal clearance boundaries of my stand without pre-approval. In 2019 i decided to have the entire property re-surveyed and plans drawn up to cover all the improvements including the 2 structures. Upon receiving the survey certificate i was shock and surprise to learn that my property encroached onto a neigbouring vacant piece of land belonging to the municipality by about 150 square meters. The bounadry walls were never move while i lived here. It must have been incorrectly or deliberatly erected initially as a mount of rocks obstructs the route of the leagal corner pegs. Whom- if at all possible could be held accountable for this error and how would i go about getting this sorted out with the minumum expenses. I was advised to approach an attourney but i'm afraid it will cost me dearly. Any advice would be greatly appreciated.