Amazonia

Amazonia was the human delegate to Peladon when it applied for membership in the Galactic Federation. She did not arrive until after the situation of Peladon had resolved, her role having been taken on by the Third Doctor. (DW: The Curse of Peladon)

Amazonia was eventually succeeded by Mavic Chen as Guardian of the Solar System. (NA: Legacy)

Behind the Scene
Marc Platt has theorised that Amazonia represents a future South American state of the same name which has, in the future, unified into a single nation.