Thanks all for the advice. Since we are spending some time in SYD on our way back, we already have the electronic visas so that is not an issue.
I showed her this thread and I think I have convinced her that carrying on is best if she can possibly swing that. She originates in Omaha and they might be a bit less familiar with checking passengers through to CHC than the folks at SFO. Not sure, but that is my guess.
Thanks!