There will be periods of time throughout the day, usually during the morning, when the Zebra are integrated with the Giraffe, although it is not always the case. At any rate, you are not guaranteed to see the two species mixed, and instead the Ostrich will resume a position in their BOMA, and the Zebra will have that fenced off area to themselves. If you do see them mixed though, always head around to the Ostrich because there is a good chance the Keepers are training or feeding them. Like
@Zoofan15 said, which I agree with, the Ostrich are not the most captivating species, as opposed to, for instance, Nyala. It would have been quite easy to bring in a small herd of Nyala from TWPZ, seeing as they are accustomed to Giraffe and Zebra, which would also allow for breeding. I can understand why Taronga made this decision however, as Ostrich are easily accommodated, and usually take a quick liking to their cohabitants. I say that last part with a level of irony, as it appears none of the animals in the savanna particularly like each other, judging by the excruciatingly long time to successfully integrate them
@Jambo Obi would not have had a chance to breed with Kioni, as he arrived as a sub-Adult and was castrated shortly after his 3rd birthday. However, the previous male was successful with Kioni, but their foal was sent to TWPZ almost immediately, and the public were not informed of any such birth. Shortly after though, the male either died or was relocated. If anyone can confirm it would be appreciated.