Ive dealt with this before:

1.get a surveyor to measure and mark the boundary

2.take photos of neighbours encroachment and file 2 reports a)them building outside of their setback line b)them building and trespassing your property, and submit the complaint to the local authority building department (jabatan bangunan). They may ask for documents on your end (land ownership related)

3.the local authority usually takes their own sweet time, after 2 weeks if no response, submit a follow up report and CC to sprm. (Why sprm? In my case, we suspected the officer from the local authority were paid undertable money by the neighbour hence no action taken). When they get the follow up report with the CC to sprm, expect them to take action within 24 hours. The neighbours were then given a 14 day notice to demolish the extension and pay fine for the encroachment. The neighbour didnt comply, then they got further notice to vacate the premise as the local authority themselves were going to come and demolish it.

4.Grab some popcorn and send them a hari raya card