Suitability for  warm temperate areas of New Zealand
Bananas are a tropical herb, and it is stretching the limits of their range to fruit them in the warm temperate areas of New Zealand, with gowth pretty much stopping in most varieties at around the 15 degrees centigrade mark. But fruit they do, as long as their needs are met. But the plants are:
-slower to produce,
-less robust,
-the flowers smaller,
-less bananas are set,
-and the most 'tropical blooded' (those with purplish or pinkish blushes to the leaf petioles) are either slow or unsuccessful.

Variety selection is particularly important. At least as indicated by sap flow, some varieties (most particularly the apparently much more cool tolerant variety 'Goldfinger') continue to function well as long as the temperature is above 8 degrees celsius, where others 'close down' at around 11 degrees celsius. Active growth in almost all varieties ceases below 16 degrees centigrade. As most bunches ripen over autumn and early winter, this is particularly important.

The banana deserves to be popular for it's productivity in a small space, it's pleasing landscape qualities, and, of course, it's delicious fruit. But the fact remains that the banana is a warm weather plant. When the cold of winter comes on, it tends to yellow somewhat, and the leaves get pretty tatty looking. In a warm winter it looks pretty good, and ripens any green bunches that had developed over summer. In a cold winter a bad frost will severely injure the plant, but it will resprout from the ground when warm weather returns. Bananas only really suceed in the warmest part of the New Zealand; but if they are tucked under the eaves of the house, their range can be extended.

It is the ideal crop for the small space gardener, as it makes best use of vertical space, is not too large, crops quickly, and the fruit are concentrated in one place-making for easy bagging against pests.

Cold hardy bananas
There is a species, Musa basjoo, the Japanese Fibre Banana, being touted as " the world's cold hardiest banana. It is hardy planted in ground to -3 degrees F. and with protective mulching, down to -20 degrees F". It is from Southern Japan, and is usually grown or the fibre in the leaves, rather than the fruit. The fruit are small and seedy, but edible.They have even been grown in places such as Canada. The trick is to much the underground corm heavily so that it doesn't freeze over winter. It re-grows vigorously when spring comes.

Time from planting to flowering and fruiting
This depends on the size of the plant you put in. A 'pup' with only a few roots from a neighbours plant will take about three years to first flower and fruit Under New Zealand's cooler conditions (vs about a year in the tropics). An established well grown plant in a large planter bag from a nursery put into very good conditions might only take about two years or even less, according to banana triallist John Prince of Nestlebrae Exotics (pers comm April 2002).

Site and care
The banana is a water loving plant, and thrives with plentiful water in dry spells and reglar fertilising. However, as long as it is fairly well mulched, it will still fruit with less than adequate water, albeit the fruit may be smaller and less well filled. Bananas are also greedy feeders-they have to be, considering the weight of fruit that is regularly removed from the clump. Spring growth is crucial. Good growth in the early months makes for larger and better bunches. The point is to keep the clump well watered and fertilised at this time. Warmth is all. Give as sheltered and as full sun facing situation as you can.
Fertilising
Use a complete garden fertiliser that has a bit extra potash/potassium in it, as bananas need quite a bit of this element for its fruit. Regular light liming may be needed on acid soils. Composted chicken broiler house manure (shavings and crap mix) is said to be particularly useful.
Desuckering
It has always been argued that the overseas practise of de-suckering should be followed. The concept is that in order to keep the resources of the clump concentrated on fruiting plants, it is best to allow two plants to fruit and have two replacements coming on. Remove all other suckers that develop. De-suckering is just another job to have to do around the place. Bananas seem to fruit just fine left to their own devices.
Pests and diseases
From a practical point of view, there are no insect of fungal problems. When bananas split on the bunch, as some varieties do in some seasons, wax eyes (in particular) will eat the fruit. Rats, too, can do some damage, but usually leave the bunches alone unless there is an adjacent macadamia crop attracting them.
Staking
Some varieties are very tall, and the weight of their bunch may drag the trunk ever more toward the ground. In such cases it is essential to eitheer stake the trunk or prop it up. Otherwise you may find it sprawling on the ground after a storm.

Planting and propogation
If you come across a banana you like, or it's owner recommends, simply get a spade and dig out a sucker. With plenty of water in the hot weather,  applying fertiliser regularly, and starting with big healthy suckers it is possible to cut your first bunch within two years of planting. Once a clump is established, there will virtually always be one or two stems fruiting. Once fruited, the stem never flowers again, and needs to be cut down. It makes good mulch for the clump.

Promoting ripening on the plant
Several techniques have been recommended for cooler climates to ensure bunches ripen -
1. Cover the bunch with a plastic bag, open at the bottom. There are special 'banana bags' that have a reflective back, and a clear blue polythene front. These are used commercially in parts of Australia, but are expensive.
2. Some recommend cutting half the bunch off to give all the resources to the first set fruit.
3. Cut off the male flower. After the bunch is set, the male flower hangs off the end of the bunch like a bell cord. It contributes nothing, and resources are diverted to keep the flower going.
It is fair to say that no comparitive trials have been done in New Zealand conditions to prove the effectiveness of these techniques.

Harvesting
Bunches are often high and just out of reach. As the stem that the bunch is carried on is going to be cut out anyway, it is often just as convenient to cut down the entire stem with banana bunch attached. But be very careful. The bunch is very heavy, and unless you  are careful, the stem plus bunch will suddenly drop under its own weight. If you try to catch it, you are likely to injure your back. Better to have two people involved, one to cut, one to ease it down as it starts to drop. If you are lopping off a bunch accessible from the ground, make sure it doesn't crash suddenly to the ground. Banana bunches can also be physically wrenched off the trunk, and as the stem tears and resists breaking it often comes down more slowly and in a more controlled manner.

Banana sap dripping from a freshly cut stem or fruit stalk will stain clothes, so be careful. Cut the bunch when the first few fruits show the first sign of color (bunches can be cut when the fruit are green but the fruit must be 'plump' to have good flavor when they ripen up).

They will ripen up very quickly once hung up inside in a warm, light place (or on a sunny deck), and have very good flavor. Winter maturing bunches - fairly typical for bananas in the warm temperate zone - take as much as three weeks longer to ripen if they are stored in a cool dark place, and their flavor is often very poor.

Links
Banana Cultivars in New Zealand JJJJJAs it is the only substantial information on banana cultivars publicly avalable anywhere, we have to give it high marks. But the authoritative guide is still to be written by those with the cultivar collections.
http://www.naturalhub.com/grow_fruit_cultivars_banana_new_zealand.htm

Banana varieties and planting instructions JJJJ About 26 edible varieties are tabularly described with a photo of the fruit in the 'Aloha Tropicals' catalogue
http://www.alohatropicals.com/musaf.html

Banana cultivar photos JJJfrom the University of Hawaii, around 15 cultivars in the archive, plus other pictures of the plant and flower
http://www.ifas.ufl.edu/~imaguire/BANANAARCHIVE.HTM

References
Matthews, Julian. 'Bananas in Hawkes Bay' in 'Growing Today' magazine, may 2002, p11.