Do not buy any wine before arriving at SYD, you won't be able to transit with it (unless it's in 3oz bottles and they all fit together into a Quart baggie).
www.dutyfreestores.co.nz allows you to join a 'f
requent buyers club', this gives a 20% discount on pre-ordered duty free items on arrivals and departure.
That web site also has good information on what you can bring in.