Sydney is a city located in Australia.
In the 1970s, a group of Daleks based underneath Antarctica used a captured naval submarine to attack Sydney. Their plan to use the city as a base from which to convert Humans into slaves and conquer Earth was defeated by the Doctor with the aid of Lt. Davis. (TVC: *Sub Zero)
On 14th July 1993, the Doctor met with 37-year-old Dorothée in a café in Glebe, a suburb of Sydney. (NA: Set Piece)
In 1995, the Doctor, Ace and Bernice Summerfield visited the interdimensional café in Glebe. (NA: Set Piece)
In 1996, the Doctor, Chris Cwej and Roz Forrester visited Sydney for a week. (NA: Return of the Living Dad)
The Doctor bought a beach-house in Sydney while his house in Kent was being painted. (NA: Sleepy)
In 2096, Sydney was an independent city-state. (PDA: The Indestructible Man)