crosvs,
Having grown up in Sydney (quite a few years ago) I can vouch for Sydney's Taronga Park Zoo. It used to be world class, I can't see any reason why it still wouldn't be. You'll obviously get to check out all of our marsupials, which are very unique to our part of the world plus all sorts of other critters from here and sundry.

Make sure you catch the ferry from Circular Quay over to the zoo as Sydney Harbour really is breathtaking. Like any decent zoo, give yourself a full day to take it all in.

Don't forget to take your camera.

Enjoy

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