The trusty OED (Oxford English Dictionary) shows the orthodox spelling is in "z" like organization, etc and in Britain Oxbridge publications enforced (and still enforce) this spelling convention. But it had been a British habit to use "s" like organisation, so much that it has now been accepted that "s" is accepted (and even preferred) in British spelling and for English variants derived from more recent British English (eg New Zealand, Australian, and South African English). It has now even been deemed "International English" standard in much of the world (International English is basically British English without its most distinctive vocabularies)
But still, writing using z is still a correct usage, but not many in New Zealand use it (some say it is too Oxbridge i.e. elitist, some say it is too American).
"...but not many in New Zealand use it (some say it is too Oxbridge i.e. elitist, some say it is too American)."
Thanks for that tip. I've made a sworn promise to Hubby that we'll decamp for New Zealand if ever Madam Hillary is elected president. I'll have to be careful of my spelling should that baneful day arrive. But what the heck! I can't really spell with a damn anyway!