If you can see brush-like marks (and possibly the mortar lines ghosting through) it sounds more like a bagged finish rather than a render. These (if they are soundly adhering) can be rendered over as the acrylic-modified renders stick like the proverbial, however you may find that some renderers won't quote on the job without including removal of it or at the very least keying of the wall, especially if the finish has been painted.
DIY electrical wiring to AS/NZS3000 - details here - http://goo.gl/9d33T (PDF file)