I had an Australian friend who said that when he was a child in Australia he was bi-lingual. His mother was very upper class English and insisted he spoke as she did but when he went to school he would immediately change over to a more Australian accent.
One of the friend´s students from an English university remarked once that if an Australian has a British accent it usually means one thing - money. which I interpreted as meaning which school this person went to...when a schoolfriend that I hadn´t met for many many years visited me recently everyone remarked that we spoke with the same accent and she has always lived in Australia.
Having said that, I was born in Australia, lived there until I was 18, I have been told that I have just a touch of an Australian accent especially on the telephone.
I was very impressed by Princess Mary, she is a very well educated and intelligent young woman and her accent is that of a very well educated woman with the influence of her Scottish parents way of speaking too.