Everything is bigger. (Insects, chicken breasts, shampoo bottles....)
People you have never met before, like shop assistants and call centre staff, will start conversations with "Hello, how are you?"
The conveyor belts at the tills in supermarkets are ridiculously short BUT the checkout operator will pack your bags for you.
Speed trapping on the roads is a lot more common (especially in QLD). You get used to driving more slowly and then, when you come back to the UK you think that you are going to die because the person who has come to pick you up from the airport drives so fast (although this could just be my FIL).
Apostrophes are missing from many shop signs and printed materials.
'Bring a plate' means that you need to take along a plate of food to share.
Depending on what state you are in, you may need to get insurance for 'ambulance cover'.
Oh, and if you have a permanent visa and you do not take up the option of becoming a citizen, your visa will expire after about 5 years. You will then need a Returning Resident Visa (which costs the same as citizenship) before you can leave and reenter Australia. Apparently EVERYONE knows this except for DH and myself. We only found out that our visas had expired at the airport when we left Australia for a visit to the UK. Cue a lot of panic and calls to the Australian embassy while we were in the UK!
I came to Australia 6 years ago and it has been one of the best things I ever did. I have moved around a lot and lived in all sorts of places from Melbourne to rural Queensland and have met some amazing people along the way. Just set your family up with Skype, throw yourself into it and enjoy the experience.