Noisy Miner
Manorina melanocephala

As the name suggests, they do make a fair bit of noise, especially when they are mobbing larger birds, which they do often as they are quite aggressive. These are one of the most common Honeyeaters in the Sydney area. The last is of a juvenile bird, which is evident from the short beak and downy feathers.
 

1-2: Camperdown Campus, University of Sydney, 03/09/2009.
3: Royal Botanical Gardens, Sydney, 13/09/2009.
4: Camperdown Campus, University of Sydney, 05/10/2009.