This was the former hippo enclosure, merged into the rhino / nyala enclosure behind after the last hippo died. There is a rhino visible between those tree trunks in the back.
I had a short thought that this enclosure could be turned into a Nile Crocodile enclosure. But 2 seconds after I thought that, I relized that they wouldn't be able to survive in Auckland because they don't have the right air or water temperatures.