You could request the time off and try to persuade the Head it falls into the exceptional circumstances category (family holidays are nolonger allowed but time can be granted for exceptional family events). If this is going to be the only time she sees her Australian family in the whole time she is at secondary school and if the dates are unchangeable then you may be able to make a case.
Being in the top set doesn't have any bearing on things but having good attendance means that, if it is refused and you have to take it as unauthorised leave, you may not get fined.
Or you could phone in sick but that would depend on how you and DD feel about doing that - you'd have to put it in writing for a 3 day absence and she'd have to lie if asked.
My sister and family will be staying (from Oz) and they have not been in the uk for 4 years now. Dd has a good attendance record and is in top set for everything so don't see their should be a problem. As I was not sure what the plans were till my sister was in the uk was unable to sort time off for dd so was planning to sent a note in to explain that she will be absent and why. Is this the best thing? Or do I just phone and say she sick which I don't really want to do.