without going into too much detail, advertised jobs at zoos are always well-received by job-seekers. In fact most zoos and aquariums receive unsolicited CVs regularly by hopeful people. Most of the applicants are not at all suitable (there are lots of "bunny-huggers" that think it would be delightful to work in a zoo and play with the animals all day!), but it has to be said that as in any job description the "skills/qualifications required" bit is more a list of what is ideal. It is still well worth applying if you lack one or two of the requirements. Having said that, Orana and all other zoos certainly can afford to be picky -- it's obviously rather a false economy to hire inexperienced staff! A balance between academic and practical is looked for (generally speaking).
As kiwipo suggests, the new job advertisements are due to the park being able to hire more full-time staff again after the lean times post-earthquakes (although its not really accurate for me to say "post-earthquakes" because they're still on-going!). There are actually six vacancies but only four are for animal keepers. Auckland Zoo sometimes has three or more jobs advertised simultaneously, and there are usually at least a few zoo/aquarium vacancies somewhere in the country (one at Kelly Tarltons as we speak, for example) but in general zoo jobs come up singly (and are often advertised within zoo circles, not always on such places as Trademe!).
Regarding volunteers, at Orana specifically the majority of them are retired folk. Generally speaking, as kiwipo also said, volunteers at most zoos in NZ/Australia usually are not involved in animal care except in a vague way. They certainly don't help clean or feed exotics as used to be common in the old days (although at smaller zoos that can still be the case). At most zoos volunteers are used for visitor interaction, gardening, things like that. They do play a valuable role in all zoos that have volunteer programmes. And of course some do eventually become paid staff (so it is a good way to start off in zoos, so long as one doesn't expect to be an animal keepers when a volunteer!).