To see loads of historic photos of Auckland Zoo, visit Search Heritage Images and type in zoo and click search. There are heaps of photos on there that I found interesting. There's some of Wellington Zoo in there aswell...
some great photos in there, of specia interest, #26, polar bear pit, check out the kid sitting on the wall with his legs dangling into the pit. #36 Lion pit as it was, now the tiger enclosure. #41/42/44 more shots of polar bear pit, that pit doesnt look as big today, dark looking bears. #43 black bear pit, bears obviously used to being fed by public.
Old aerial photographs can be viewed at Auckland Council GIS Viewer. Search for Motions Road on the right and then click the Tools icon on the left and select Historic Aerial photography. A box will appear on the right which will allow you to drag an arrow through the dates.
I have found some more old photos here - auckland zoo | page 1 of 2 | Items | National Library of New Zealand and here - DigitalNZ