SYDNEY - Thousands of Australians were Saturday ordered to evacuate their homes in Sydney’s northwest and elsewhere in New South Wales state as heavy rainfall flooded rivers and waterways. State Emergency Service Commissioner Murray Kear said that 3,500 people were subject to evacuation orders as 75 percent of the state, the most populous in the country, was affected by flooding. Days of heavy rain after a damp summer have seen Sydney’s Warragamba Dam, the main source of the city’s drinking water, reach capacity for the first time in 14 years — prompting the water to move over its spillways late Friday.