We have several plants here and get fruit most years

We had one on the  patio as a kid in Westport.
Where do you get the seed? I understand that commercial bananas are sterile these days

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I have tried with the Ladies Fingers ones...they grow too big, of course.!I think there would be some dwarf ones...think they were called hamoa and from Fiji.
I would put them in a conservatory, with a concrete floor, and a suitable tub and they should be fine. Lots of watering needed. Also, rotate the plant daily by 90 degrees, so as to even where the light falls on the leaves.
The other option is to use the common larger types, like I do,  and recycle them outside into the garden before you cannot move them out the door! Then replace with a convenient, younger, one.
[I know, I know, some will this is a parable for modern living]

Yes, vegetative reproduction is the norm for gardeners. Some produce seed- eg Ensete - and grow readily from that.
I imagine that they are also reproduced through fancier, scientific means, as well.

Westport would be favourable for growing basjoo: looks like ladies fingers but produces inedible fruit. Japanese used it for the fibre. It is the most cold hardy one I know of.
I also think you could grow the giant Ensete and could experiment with Ladies Fingers. Don't know if they would fruit but if you have a favoured spot that avoids frost and wind, worth a go.

As to obtaining them: garden centres  locally (New Plymouth) sell various species, as do many further North.

When my ensete produces seed, I can send some to you, Tony, but it may be a year or two away.

Look around: you may find someone has a plant or two growing in Westport. The young suckers can be divided off, much like canna lillies are divided.

OK: today's update.
I begin with the the disclaimer/EULA- or as the politician's minder said at the media conference:
"Here's the statement, now here is the denial!"
I am not a horticulturist, nor any kind of botanic expert. Just a gardener who likes growing certain plants outside their normal range. There will be experts around in the propagation and cultivation of bananas and I imagine that they will reside in the Far North.

Right. I first grew bananas in Singapore: not much is required to do that.
Back in NZ, where I lived in Northland, I noticed that in the Far North there were some bananas growing, so I transplanted some and grew them. Not too difficult to do and fruiting was reasonably reliable. People were experimenting with different types and it wasn't difficult to obtain young plants.
Then I moved back to Taranaki. There were some basjoo bananas growing on the farm at Cape Egmont, so I started planting more of them around the garden. I also had some others that I transplanted from up north.
Because of the constant wind- the awful, dry shredding SE, shelter was of essence for the edible fruiting ones.
Anyway, I moved into town and now have a quarter of an acre around the city's original green belt.
Things are a lot easier here: the wind is much less and edible fruits have been a regular bonus.
I primarily see bananas as a landscaping option, with the bonus of fruit.

Rule 1: Location.

As warm as possible. As sunny as possible. Plenty of regular rain. Sheltered from strong, cold winds. No frosts. (NB: this  is the ideal: they will survive light frosts- I have seen that in inland Northland; becomes a blackened stump and rises again)

Rule 2: Soils:

ideal is volcanic. As rich as possible, these plants are gross feeders.

Rule 3: Climate: Study your local climate/micro climate. For most of us, we are trying to grow plants well outside their natural range. (Actually the whole of NZ is outside the natural range, but the Far North is closest in terms of matching requirements of heat).
As a guide: for me: usually each year: Sunshine  just under 2,200 hrs, Rainfall: 1,400mm and reliable. Screen frosts nil .Groundfrosts: 2 or  3 and only for a hour or so at the most. Wind: probably around 20 km/hr mean wind speed. Summer: 22 degrees day/14 nights. Autumn: 20/12, Winter: 14/7 Spring : 17/10

In my case, the low diurnal/ annual range is good, the lack of frost is good.
But my Summer maxima are   lower than ideal: The Far North, Auckland, Tauranga, Gisbourne , Napier would be better off. But then those places have a greater chance of drought.
Also, I have had to provide shelter, considerable shelter to moderate the dry, cold, SE wind that comes from inland: less of a problem in other regions.

(to be continued, with some pix, and specific to my location.)

Yes, I would expect some would do OK there

Herewith several young plants ready to be divided off from the colony. Dig down and make sure that you have plenty of root. They divide much the same way as a canna lily does, the main difference being the size.

In the shoutbox was:

beteljuice:
I have one black and rotting ....
MUSA BASJOO - didn't like several nights < -12C, although I think it's still ticking over

ato2:
That's the hardy one of the family. The Japanese wrap them in straw, I think, to survive the winter. <-12 is quite an ask for banana, but it may come back!

Although a couple of months too early, the beteljuice cut down the rotting psuedo stem to appox. 24cm and put a large bell cloche over it to try to warm up the soil and 'dry' the rot.

Three days later and 10cm of center leaf stem - so cloche off and put in dull corner of the (cluttered) potting shed until the nights are warmer.

No sign of the couple of 'peepers' I had in cardboard tubes filled in with perlite. Have to wait and see 

Thanks for that, Weather Display. Yes, ripening in the way you do is the way, I reckon. Some people in New Plymouth are keen on the plastic bags but I don't bother.

The wind protection helps... that photo was taken just after 2 days of those awful winds last week.
Having said that, they survive wind, but the leaves get rather tatty as they tear.
Plus, I saw a hedge of low growing species used as a wind break in Norfolk Island.