I am no expert, but also have a diesel E53 and I understand that our diesels are fitted with a Diesel Particulate Filter which can clog up if you only ever do short urban trips. Again I don't know a lot about this, but my understanding is that the DPF needs periodic sustained driving at more that 2000 rpm to burn the soot out. You can get the DPF cleaned, and or replaced, but as with all things BMW this is probably not cheap. If your car only does the smoking bit the first time you push it after a period of only urban crawling then I suspect this may be the cause, and it is probably nothing to really worry about.

I know that there are diesel injector cleaner products that you can put through the fuel system, but they say that Diesel injectors are much less prone to clogging than petrol injectors as they run at much higher pressures. I think it is unlikely that your problem is caused by a dirty injector.

You also should opt for low sulphur diesel if you have that option in Oz.

__________________
X5 2006 3.0d M57N
GA6HP26Z Tranny
180,000km