This is going to come down to what's a "real" castle. Wikipedia opines:
A castle is a type of fortified structure built during the Middle Ages
predominantly by the nobility or royalty and by military orders.
Scholars debate the scope of the word castle, but usually consider it
to be the private fortified residence of a lord or noble.
So it's got to be old, fortified and a residence for nobility, which means pretty much everything in New Zealand and Australia is right out, since they have neither medieval structures nor nobility. Maori pā were fortified settlements not limited to nobility, so they don't quite tick the box either.
Probably the closest is thus Japanese castles, in particular Himeji, which looks like a castle, has an unquestionable pedigree (founded 1333, home of arguably Japan's greatest shogun Toyotomi Hideyoshi, etc) and is easily reached from Osaka. There are many other castles in Japan, but most are concrete reconstructions of varying degrees of fidelity (Osaka Castle itself is, sadly, at the less faithful end) and further away from major airports.
Tibet and India also have various royal castles/fortresses, some quite formidable, but these are further away from NZ.
Honorable mention goes to Indonesia, which has a series of royal palaces called kraton, some of which were fortified. However, as far as I can tell all the fortified ones are in ruins now. There are also various Dutch East Indies era fortresses in the country, but they don't really qualify as residences of the nobility.