Unlike others, I would strongly recommend you stay well away from the major population centers and major schools, rather seek out the smaller places and aeroclubs. As has been suggested, Mainland in Dunedin would be highly recommended -the two major instructors there are both close personal friends and CP's I flew with over many years. The downside (if any) of Mainland would be perhaps the predominance of CPL candidates, that may overshadow your own efforts... having said that though, the staff are all superb and the training peerless.
Another well-worth considering IMO is the Wakatipu Aero Club in Queenstown. You'll not only be extensively exposed to mountain flying -something others can offer only at considerable effort and most likely expense -but you'll be mingling with active commercial pilots daily both professionally and socially. Were I to have my time again, Wakatipu would easily and definitely be my choice.
Weather will most likely be far less of a concern than you think. What you will be exposed to will be invaluable real-world experience.
As others have said, the exams will need to be done in NZ and the license converted to JAA on your return. Hours will be a minimum of 50 for the issue of a full license, but the small additional cost in hours terms will be more than adequately compensated in experience and skills few of your peers will possess.
Go for it. You'll have a ball.