Sydney-A week with a 4.5yr old-too long? What to do?

[whooosh]

DD and I will be spending 6-7wks in Melbourne on a 3mt sabbatical trip.We had planned a week in Sydney but am concerned this may be too long and also too expensive-it will be at the end of March.
Should we stay in Sydney or head elsewhere?

Is Brisbane worth a visit?

Nope. There's plenty to do (if you can get your head around the fact that there are hundreds of things to do in Australia, but thousands of things to do in London), - well it is pretty so you can wander around.

Lots to do in Sydney (lived there for 5 years). The zoo is lovely and offers a stunning view of the harbour. The aquarium is wonderful too. I bet your DD will love going on the ferries too. Lots of beautiful beaches. It's a very 'walkable' city and lots of lovely cafes etc so you won't be at a loss of things to do. You can also do a day trip out to the Blue Mountains, which is beautiful. I doubt you will save that much choosing one city over another... unless you are planning to stay way out! Of course, Sydney will be slightly more expensive but a week won't make a huge difference.

Brisbane is slightly dull in my opinion. If budget allows you are much better heading up north. Cairns is terribly touristy but just a bit further north there are some lovely places such as Palm Cove, Port Douglas etc... There's also the Great Barrier Reef (water is so clear your DD will be able to see the fishes without getting wet!). Have fun! I am so jealous!

Do lots of walking - Sydney very walkable imo.
Go to the zoo
Take a bus to Bondi or Coogee beach
Go to the opera house and have a drink / lunch in the cafe
Take a boat ride around the hrabour
Go down to Darling harbour for lunch, the aquarium and a mosey about

Sydney is quite child friendly and the weather is so good most of the time it's a pleasure just to be out and about

Thanks everyone-I love Sydney but have only been when childless.

Any other suggestions-blardy exchange rate is a nightmare

Can also recommend the BLue Mountains, although maybe not super interesting for the 4.5 year old. Also the botanical gardens. The monorail is fun and you can just go round, and round, and round and round for hours! The centrepoint tower is fun too and has an interactive ride (well it did last time I went up it!). There is also an art gallery in darling harbour that I can't remember the name of, but a bit like the science museum and good for kids. The queen victoria building is good for shopping and has a clock with little people that come out and dong the time every hour. My kids thought it was worth putting up with my shopping for!!