There's an age above which apparently the accent doesn't change. That's not to say people aren't going around faking accents!
A mum at the school gate asked me in a condescending voice around 2 years ago (we'd been here about 4 months) how I was 'coping with the weather'. She has a fake accent and had been here 2 months longer than us. :) We are both married to Australians. I said "Same as everyone else. Surviving". I am happy enough with life to speak with the midland / northern / Norfolk / Queens English mix that my life has taught me from living and working in different places.
I know people exaggerate their accents cos you meet people who've been here around 30 years and they still sound as though they have just come on holiday or not long been here. They appear happy enough not to need to change. :)
My kids are a mix of accent but always have been cos DH is Australian and they are surrounded by Australian family. I don't think they will ever probably have pure Australian accents. Adelaide isn't a strong accent anyway so that may account for it.