Mainly city driving, not a hard driver always set cruise to speed limit i'm in. Bmw said nothing when oil changed

You are likely not getting it hot enough to burn off the carbon (coke) deposits that have built up over the past 20,000 km. They ignite the residual fuel, causing run-on. If it is just a second, there isn't much to worry about, it is just an annoyance.

You could try using a one-time combustion chamber cleaner, but it is a toss-up as to whether the cure is worse than the disease.

We would call it run-on in North America, not sure about Oz. Overrun is defined as the engine speed being higher than the throttle setting, ie when you are going down a long hill and using a lower gear to slow your speed.

Sounds like it might need a good 'fang' in the country on a nice quiet road. I've never owned a car or 4WD (diesel or petrol), or a motorcycle, that didn't respond well to a good hard country run once in a while.

So now I know another phrase for an Italian tuneup.

Yes, as in Juan Manuel Fangio. I like "Italian tuneup", that's new for me too so we have both learnt something.

Another question: What diesel fuel do you use? B5 or B10 by chance?

standard diesel from regular servos, don't know what type they sell. we don't go to the country too much but have been regularly going up and down the freeway cruising at 100 should that be enough? did go to Toodyay a few weeks ago so could go at 110. yesterday did a 32 km round trip using freeway, cruised at 100 where possible but when we arrived it did it, so it's not like I was crawling around and getting it. Only does it on trips of 30 mins or more, speed is varied 60/70/80 up to 100

Just be aware that some of the mainstream servos now have B5 as a default diesel fuel. This may not be the problem but just check when you are filling it. It should be noted on the bowser that it's B5. I'm not a fan.
And, just on your driving, how often do you give the engine full throttle and work it hard (to max revs) under a load, ie., up a steep hill?
JCL's suggestion about a build up of carbon deposits is the likely explanation and is what I was trying to get at when I asked about your driving regime. Ultimately, the problem most likely lies somewhere between your driving regime, your driving style and the fuel you use. It could be outside of those areas but these are the "usual suspects".

Don't tend to use full throttle very much, will give it a try. Strange it does not happen every time.

Obviously you need to pick your time and place for any full throttle driving! Safety and legal issues take precedent over engine running issues at all times.
That's why I mentioned a quiet country road. Preferably with steep hills after slow corners. I guess that you don't have those sort of roads over near Perth like we do on the East Coast.

As for why it happens some of the time (you say after 30 minutes of driving?) but not all of the time, it seems to me that you might be getting it hot enough for the carbon deposits to ignite left-over fuel but the engine isn't getting hot enough to burn them off. As I mentioned, I'd also pay particular attention to your fuel as well.