£299 in extra taxes and fees does not sound right at all. You changed SYD-AKL (int'l) for SYD-OOL (domestic). You have already paid the Au$55 Australian Passenger Movement Charge (AU) and Au$29.65 Australian Int'l Passenger Service Charge (WY) for your original int'l sector. You changed AKL-xSYD-HNL to OOL-xSYD-HNL which means you are subject to both these taxes (AU and WY), and as you have already paid this then there is no additional charge. The only additional taxes will be Australian domestic taxes for SYD-OOL-SYD. There is no APD equivalent in Australia for domestic or int'l flights. As you are no longer flying from AKL then you should be due a refund in New Zealand taxes/fees and charges.