I don't have exact numbers, but a large swath of immigrants came to Hong Kong after WWI and especially the Chinese Civil War. For the most part these were wealthy, pro-KMT emigres who brought large amounts of capital into the city (as that's why they were let in, as opposed to the poorer KMT emigres who fled to Taiwan). I believe the population trebled after the war, leading me to conclude that a very large amount of the population are not "native" to the Hong Kong area.

I believe that most people in Hong Kong arrived after the British took over, because under British rule it became a major commercial center and that attracted mainland immigrants.