Hi Sylvia,
No need to have "trepidation" about your holiday to New Zealand. You will have a fantastic time, see exciting sights and do things you have never thought of.
I did NZ in 2012 and with others on your JY holiday have a fantasic time. Recommend you do all the activities you can whilst there. Do try the Helicopter ride at Franz Joseph Glacier and the flying boat ride at Rototora to see the volcano. Queenstown is very good place to visit for a few days. Do try and go up the skylift to the top of the mountain. The 4 hour journey to Milford sound might seem a long time but is well worth it, some spectacular slights including the tunnel through the mountains.
I found NZ to be cheaper than Australia whilst there. Food is excellent and as stated you are in good hands with JY.
Have a fantastic time.
Eric.