That is a tough question to answer. Since New Zealand has no character design whatsoever it is hard to draw him in the strips. Though the reason with Australia is unknown. We will just to have to wait for Australia's and New Zealand's appearance.
Might I interject? On the back of volume three, New Zealand was shown in SD form with her/his pet sheep.
While she/he has no 'complete' character design, from the recent chibi we can deduce that while she/he looks to be of indeterminate gender (although it's probably a 'him,' just an androgynous 'him'), she/he has medium-length, slightly-curly (probably light-to-medium-blond) hair, and most definitely Arthur's Iggybrows. There also seems to be a consensus that he possesses green eyes. (Sample picture in addition to Wikia's own: http://hetalia.wikia.com/wiki/File:Newzealand_chibi.jpg & http://danbooru.donmai.us/post/show/726246/-_-absolutely_everyone-america_-hetalia-annotated-) This tendency to anthromorphize nations without giving them complete illustration can also be said for Monaco, Tibet, and possibly even Seborga and Wy. (FFFF EDIT: Nevermind. I didn't see the Character Cards, which show colored SD designs for Monaco, Seborga, and Wy.)
Also, while this is impossible to confirm, it wouldn't be hard to think that Himaruya would operate whimsically rather than logically when drawing his comics. There were definitely more than eight countries who fought in the Great War, but the focus tends to lie on the eight main characters, no? He hasn't been drawing as much as before, either (maybe because he's still studying design?).
Another possibility is that Himaruya uses England/UK/Arthur Kirkland to represent the majority of the fighters under the former British Empire. Australia went through multiple stages of independence, but he and New Zealand have not -completely- separated from Britain and the Commonwealth. A slight deviation from this, however, would be Canada/Matthew: while he hasn't gotten as much screen time in the series, and is typically considered a more 'minor' character, he -was- still considered a dominion under the United Kingdom during the Great War. It's possible that his introduction into the actual manga strips might be because of his relationship with America--maybe even act as a possible foil to Alfred's personality.
/long answer is looong/