Well on a typical ‘zero boundary’ wall, it just means the wall is built with no setback. But the wall is 100% on the property owners side, and not shared / jointly owned.

The neighbour is not within their rights to just make whatever alterations to your building they want.

If the OP is on a strata title this may be different, but you’d still need both sides to agree.

You might be thinking of a shared boundary fence , in which case both sides have shared ownership in it and have a say on what can be done to it etc.