@prayingfortime:
Australian state: it's not Staat that is meant here, but Bundesland
Australia
is the fifth continent. The name Fifth Continent probably originated with Jules Verne's 20,000 Leagues under the Ocean with the phrase:
"Then this new island will be joined later on to the neighboring groups, and a fifth continent will stretch from New Zealand and New Caledonia, and from thence to the Marquesas."
found here:
http://www.thefreedictionary.com/New+ZealandThe postulation only works if the islands combine to create one large piece of land = continent. As long as they remain islands, they are not communally a continent.
Neither Oceania nor a combination with New Zealand are elements of the fifth continent - what you might have meant is Australasia:
http://www.bartleby.com/61/87/A0528700.htmlDown Under is classically, and in most people's eyes and understanding, Australia and Australia alone.
Dictionary references to New Zealand support evidence that the term is/was used for New Zealand as well, but, in my experience, no-one from New Zealand would refer to him/herself as being from Down Under, nor would they so be referred to by others, for fear of being mistaken for Australians...
"I come from a land down under" is a sentence in the style of fantasy novels and stories: "I come from a land far away across the ocean", and makes no concrete references to a single land among others making up the fifth continent.
In spite of this, I consider your suggestion suitable