Hi Setha,
You may recall you visited me years ago when you came to collect organic wheat straw. I'm growing the bananas further down the hill below the house. The site is reliably frost free.
Bananas are a bit different from most other fruit trees in that the Tbase is also the sapflow ceasing point, and once banana sapflow stops then sap coagulation occurs which damages the fruit. There are a few bananas with comparatively low Tbase of around 11C but most are 14 or 15C or even higher for the truly tropical adapted ones. In practice the sapflow matching Tbase means for every hour below about 11C (or 15C for some bananas) there will be damage to the fruit, which gets correspondingly large as the hours below Tbase increase. In subtropical climates winter ripened fruit is always inferior, and cooler climates are that much worse off.
For home growers in NZ a site that has at least moderate summer heat and never falls below 4C is warm enough to produce fruit like Maureen has grown, which is still great eating for backyarders.
Usually a moderate frost will kill all above ground biomass, but it will come away again in spring or summer heat is sufficient. Ideally bananas need 27C average daytime highs all year around, but in the subtropics they can still be commercially viable in places that only have 27C (or higher) daytime highs for 8 months. This is why NZ will never have a commercial banana industry outside greenhouse production or short of extreme global warming, as there is not a single official climate station in NZ that has 27C mean daytime temperature for even one month of the year.
There are banana plantations in Northland, but they all experience winter chill injury and far lower producing than would be considered viable in any other country. Most of Northland has a mean annual air temperature between 14-16C, with a small coastal section of the Far North between 16-17C. Hawkes Bay is mostly between 11-15C MAT (colder in the highest hills obviously). In Australia the coldest commercial banana growing areas have MAT around 19-20C, with the main production area being around 24-5C (Cairns, Innisfail etc).
We can grow bananas in HB in the warmest sites where they need perfect shelter, all day sun, no frost, heaps of mulch, lots of water over summer, and as much potassium as possible, after which they are capable of producing a moderate amount of sub-commercial quality bananas that are still pretty good eating.