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.