Hi,

Early May a bit cool, Mid Sept will be better.
Oct-Nov way better, but more expensive as well.

If you like to do some homework, self driving is a better
and cheaper option. 14 day can cover both north and south Island.
I would say split 8-9 day for SI and the rest NI.

I did it with some friend, and my own family, wife and 2 kid,
one is 6, another is 4 year old. Basically, you can rent a sedan
car and stuff the luagguges and others into car boot. Prepare to get
some infor on travel NZ on internet and book the hostel in advance.

Try to get airfare land in Auckland and departure from Queentown.
( if the cost is the same for land/depart from same city)
You will save abit of travelling cost by not repeat the route.

Not much problem or boundaries at any month. But as highlighted by Munkeyflo, from our point of
Malaysian view, we prefer to travel in a cool wheather because over here was hot whole year round.
For me i prefer temperature range 5-15'C. My last visit was in early May 2012, temperature 0-10'C.
It abit cool, but ok for my family and me. If you travel whole NZ, then will be ok, since NI warmer than SI.
In early May, you can see some snow  cap on the the mountain already.
But avoid July, some road will be close due to snow, and shorter day time.
For the attraction place, route visit , cost and tips etc.
i had share with the forumer here, please
refer back to pg 24-26,  post date: April to June 2012

If you brain  enough to read through  whole thread,
70-80% of NZ travel infor can be getting here.

There are not many choices for flights in and out of NZ. The two main choices are MAS (direct KL-Auckland) and Jetstar (KL-Auckland transit in Singapore for 3 hours or so).

Obviously Jetstar will take a few more hours to arrive your destination compared to MAS since you have to transit in Singapore. Luggage will all be checked in directly so you don't have to worry about that. Jetstar is like Airasia, you'll have to purchase luggage weight, food and/or entertainment, water is free to refill if you have a water bottle though. MAS is a full-fledged carrier so you'll have free flow of food and drinks on boards, etc.

As for traveling domestically around NZ, try Air NZ or Jetstar. Both of these airlines also give good pricing when they have promo. I find Jetstar have more promotions than Air NZ though. All tickets can be bought online so check both airlines when you buy domestic flights.

North to South island by car, you'll need quite some time to drive considering you'll be stopping by tourist attractions and what not. Since there's no direct flights to south island from KL, you can take a domestic flight from Auckland to Christchurch/Queenstown when you arrive. If you have the time to spare, take the drive down from Auckland to South Island. Check with the car rental company beforehand, most of them have branches in the main cities and you can return the rental car to their other branches, i.e. rent from Hertz in Auckland and return to Hertz in Christchurch. Then catch a domestic flight back to Auckland for your flight back to KL.