New Zealand English: Possessives

Is it Charles' book or Charles's book?

Is it right or wrong when we see signs in supermarkets selling banana's and apple's?

And, finally, if the Jones family own a house. Whose house is it? The Jones’s house, the Jones’ house or the Joneses’ house?
                
Let’s go back to the basics to work it out.
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Singular possessive
With nouns that do not end in a s, we use an apostrophe and an s (‘s) to indicate that something belongs to someone or something .
                The girl’s books – some books belong to one girl
                The dog’s bone – a bone belongs to one dog

An apostrophe never indicates plural, so when you see "banana's and apple's" on a sign, it is wrong.
 
Singular possessive noun ending in s
But, with a singular noun that ends in s, x, or z sound, the final s may be left off if it makes it difficult to pronounce, though it would be preferable to rearrange the words.
                The fox’s hole – easy to say so we can leave the s
                The catharsis’ effect – difficult to pronounce with an extra s, so we can leave it off  

Names 
With personal names, the ‘s is usually used, but the final s may be dropped if it makes it difficult to pronounce.
                Charles’s book
                James’s
                Nicholas’
                Lord Williams’
                Jesus’s  – this is the non-liturgical use, but use Jesus’ in archaic form
BUT        Erasmus’ – classical names traditionally use an apostrophe only

Plural possessive
With plural nouns that do not end in s, we add an's.
                The children’s school – the school belongs to many children
                The women’s race – the race is for many women
 
Plural possessive noun ending in s
With a plural noun that ends in s (as most do), always use an apostrophe only to indicate possession. This also helps pronunciation.
                The foxes’ hole – the hole belonging to the foxes
                The girls’ books – the books belong to more than one girl
                The ladies’ room – the room is for more than one lady

Possessive of plural last names
Firstly, the plural of a last name should never use an apostrophe. If the last name doesn't end in an s, add an s for the plural (even if it ends in a y).
                The Smith family are the Smiths.
                The Kennedy family are the Kennedys (not the Kennedies)

If the last name does end in s sound, add an es.
                The Jones family are the Joneses
                The Edwards family are the Edwardses     

So, using the rules above, to indicate possession, we add an apostrophe to the plural of the last name, but we don't have an extra s, as the rule for plurals states to use only an apostrophe - besides which, it would make it very difficult to pronounce.
                 The Joneses' house
                 The Smiths' house
                 The Edwardses' house

Some other examples of idiosyncrasies of possessives:
                In two days’ time
                It is yours and this is theirs – no ‘s
                The Prime Minister’s job – compound phrases only have a final ‘s
                My mother-in-law’s house – compound words only have a final ‘s
                Mum and Dad’s house – the house belongs to both of them
BUT        Mum’s and Dad’s shoes – two pairs of shoes, one for each of them
                We are at the doctor’s
   ​
So, to answer our questions above, it is Charles's book (though it could be Charles'), it is wrong to use an apostrophe for plurals (so banana's and apple's is wrong), and it is the Joneses' house.

​What next?
If you have written a book in New Zealand English and this all sounds too overwhelming, I can help. 

I am a copy-editor and proofreader based in New Zealand. My business, Clearlingo Editing and Proofreading, caters to all writers of fiction and non-fiction books. I can discuss with you where your book is at and what you need to do next.

For more information on how I can help you make your book shine, please contact me on: www.clearlingo.co.nz/contact.

I would love to hear from you.​

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External Sources:
Grammarist (2009-2914). Last names: Plural and Possessive. Retrieved from http://grammarist.com/style/last-names/
Oxford University Press, (2016). New Oxford Style Manual (3rd ed.). United Kingdom: Oxford University Press.