In this postcard posted in Canterbury on 21 July 1908 (I assume) it is marked that:
This space may be used for communication for Inland Postage and Foreign Countries, except Japan.
The online glossary from the New Zealand Philatelic Federation has this to say on the subject of postcards with divided backs:
Divided back The earliest postcards carried the recipient’s address and postage stamp on one side and the message was written on the ‘picture’ side. Such cards are known as undivided back postcards. In 1902 Great Britain introduced the divided back, a picture on one side and a divided space on the other side for both the recipient’s address and sender’s message. The transition from undivided to divided back took many years as postal authorities around the world adopted similar standards; 1904 in France, 1905 Germany, 1907 in the USA, and so on. In New Zealand this depended on where the postcard was being sent to. Divided back cards would be sent to Australia from Jan 1905, to Italy and Holland from Dec 1905, to Belgium, Canada, Mexico and Thailand from March 1906, to most countries except USA and Japan from April 1906, and to most countries except Japan from October 1906 and to just about everywhere from Dec 1906. The reason for this is that the receiving country decided if divided back cards would be accepted or not. This helps to date unused postcards. Cards before these dates have undivided backs.
Presumably, that postcard had been printed between October and December 1906, when that restriction was in place, but not actually posted until July 1908.