My children were 4 and 5 when we emigrated. Eighteen years on and I don't think they have Australian accents at all. However they do use Australian phrases, terminology and words. What they say and how they say it are two different things entirely.
Adelaidians have only a slight accent. It's barely noticeable and is very similar to a Home Counties accent. The Victorian accent is more noticeable, and an ocker NSW bush bunny accent (which some of my rellies-in-law have) is almost incomprehendable.