One more flat rate shipping tier

My client (in Sydney, Australia) is requesting a three-tiered flat rate shipping arrangement. Tier 1: Sydney Metro, Tier 2: Australia-wide, Tier 3: International. How can I go about modifying the flat rate shipping method to achieve this? The Sydney Metro postcode range is 1000-1920; 2000-2249; 2555-2574; 2740-2786. Australia-wide would be any other postcode with AU as the destination. Have there been similar requests for modification of flat rate shipping?