Best way is probably to disassemble them and send them out for rechroming. I'm not sure about Australia, but here in California chrome plating is getting pretty pricey (enviromental stuff). Cheap way is probably to paint the reflector with some of that turn signal chrome paint. Not really the greatest, but it will only cost you what ever a can of spray paint costs you over there.
Its not because of rust under the chrome, right? That happened to me, and it wasn't fun to replace... I would say the answer depends on your budget. There are ways to counter the dimming by just upping the power of the lights (brighter bulbs, pulling the diffusers) that might just be a lot easier. Also, the respray is a good idea for a cheap solution. FYI, if you chose to replace them, the housing will be probably $25-30 apiece at best unless you can find them in a junkyard (about $11-12 apiece there)
I haven't had a chance to try this yet, but I'm told you can heat up the housing which softens the seal between the glass lense and the reflector backing, then you can wire brush off the peeling "chrome" finish. I think the light diffuser can be drilled out and pop rivited back in later. You can buy chrome-like spray paint at the local hardware store, or try http://www.eastwoodcompany.com item # 10005 "Reflective Aluminum Aerosol"
The original reflective surface is chrome paint and not chrome plate like I had expected. The original reflective surface has three layers of paint. One coat of clear, followed by a coat of chrome paint, followed by a final coat of clear. The dingy brownish looking crud around the rim of the glass lens is actually the clear topcoat that has seperated from the chrome paint over the years. This is usually what causes the housing to leak and cause the finish to become ruined from moisture. You can easily remove the lens from the housing if you soak the assembly in near boiling hot water for a couple of minutes to soften the grey colored adhesive. Use a small flat bladed screwdriver to "carefully" pry the lens from the housing once the adhesive becomes softened. You might try powdercoating the reflective surface with some of the newer reflective coatings like "Ceramachrome" or "reflector chrome".