From the zoo's 2004-5 annual report (http://www.wellingtonzoo.com/net/annual-reports/annual-report05/life_sciences.html): "The three male and one female thar were euthanased because they'd been exposed to Johne's disease. This disease is common in hoof stock and causes painful chronic inflammation of the gut. The two female barbary sheep who shared the enclosure were also euthanased. All other Zoo hoof stock were tested for this condition and the results were negative."
(Just as an aside that second report also mentions approval to import leopards and wombats, and submitting application for approval to import maned wolf.....)
I can't find anything on aoudad still being held privately in NZ though.