In my dream master plan for Auckland Zoo, I see the Hippo River area being completely renovated and rethemed to an African Rainforest. All species here will change, and the area will be expanded to include Elephant Clearing (elephants will be housed at the Open Range Zoo instead). Gorillas will replace elephants, and a host of other species will move in, notably Mandrills and Bongo will replace the Baboons and Cheetah (also moved to ORZ). Finally, the Hippos (moved to much larger exhibit at ORZ which features grazing!) will be replaced with Pygmy Hippos.
Slightly more realistically, the hippos still have a fair few years left, and it will not be for some time that the zoo will need to decide what to do with the species. When they only have one hippo left, I highly doubt they will consider sending him overseas (or elsewhere within NZ for that matter). At that stage, they will have to determine whether to import more hippos or not. In all of Australasia there are only four zoos holding a total of 13 hippos, and new imports are urgently needed, although currently prohibited. They may decide that pygmy hippos would be a more suitable species, they certainly would be much easier to manage in a small urban zoo, and could be more easily exchanged between zoos.
Personally, I would like to see hippos maintained in NZ, they really are a fascinating species. However, as they are better suited to open range zoos (e.g. TWPZ and WORZ), until such a zoo is established in Auckland (so they could have year round outdoor access), I don't think more should be imported.