Sydney was a city located in New South Wales, Australia.
In the 1970s, a group of Daleks based underneath Antarctica captured a nuclear submarine and used it to attack Sydney, destroying Sydney Harbour Bridge. Their plan to use the city as a base from which to convert humans into slaves and conquer Earth was defeated by the Third Doctor with the aid of Lt. Davis. (COMIC: *Sub Zero)
Sydney was one of ten major cities across the world, all of which had underground railways, where Coldfire Construction built a reactor within a period of 18 months as part of a plot by the Slitheen to drain the Sun's energy, operated by at least one Slitheen disguised using a skin suit made from a local. At somepoint in 2008, after Sarah Jane Smith foiled the Slitheen with the help of her friends at Park Vale Comprehensive School at Ealing in London, she contacted UNIT so they would clear up the Coldfire sites. (TV: Revenge of the Slitheen)
In 2010, Sydney was plagued by chronal waves caused by the SKARDIS which released prehistoric creatures into Sydney and aged the passengers of an aircraft greatly. The Tenth Doctor and Majenta Pryce investigated with UNIT. The last chronal wave the SKARDIS caused before it was destroyed froze parts of Sydney in ice from the Ice Age. According to the Doctor, it would take Sydney years to rebuild and recover from this new Ice Age. (COMIC: The Age of Ice)