Do I need or should I get a boundary re-survey to build/erect a retaining wall or fence on my land boundary? Do I need a repeg when planning to build/extend on or near my property boundary line?

Yes, most definitely. To avoid costly boundary disputes with your neighbour whenever a fence/wall is replaced or newly constructed you should have a licensed surveyor re-establish the boundary and provide a re-peg certificate (Regulation 25A) as proof for your records.

If a building is built over the boundary you will be required to fix the encroachment issue and this is generally an expensive exercise – multiple times more than a survey to get the boundary line marked correctly in the first instance.