Putting the Mac for sale on their site constitutes and offer, placing an order constitutes acceptance.
It's the other way around. The offer in this context is made by the prospective customer, i.e. to buy item XYZ for ABC dollars. The merchant then accepts (or rejects) the offer. A merchant doesn't have to sell something, even at the price they have it advertised, as long as they aren't discriminatory about it.
IANAL, but if you had ordered one of these and Qantas charged your credit card before reneging, then I believe you would have a legitimate legal complaint, since taking your money is the acceptance. In your example, the merchant might have decided it was easier to take the hit on a small number of sales rather than risk fighting it.