I have visited NZ six times (3 times by cruise ship and 3 times doing a fly/drive). South Island my absolute favourite, stunning scenery, friendly locals. Have you considered using their train system from Auckland. They have finally re-opened after Covid shut-down.
My next NZ trip will be train from Auckland to Wellington, then ferry across Cook Strait to Picton, train from Picton to Christchurch and finally Christchurch to Greymouth on the west coast of the south Island on the TranzAlpine. You can also hop off the train wherever you want and spend a few days there. Check out railnewzealabd.com.
You could then go on a cruise from Auckland back to Sydney via Tasmania. I am suggesting the rail journeys if you would feel uncomfortable driving on the "other" side of the road.