Actually, this is very common in NZ. Many people plant the trees and assume it is the boundary and only realized it when the actual survey is done. They also normally build fences on the tree line without surveying.
Surveyors are independent, they won't simply listen to any side and they are governed by the Society of Surveyors, so they will do marking on the real legal boundary.
Remember there is a clause in the Sale and Purchase Agreement some sort like that:
" the vendor is not obliged to point out the actual boundary (as they are not sure) and the buyer should engage the surveyor to do so if they have a concern on the actual boundary of the property" (not actual wordings)
You have not lost any of your land, the land is still the same as your title, it is that you encroached your building on the neighbour's land, which they are entitled to take it back.
The clause on the Sale and Purchaser Agreement actually put the burden on you on the boundary, and if you didn't engage any surveyor at the time you purchased the property to point out the boundary, you basically have to bear the consequences as you face today.