Andustar was roughly rectangular in shape and size, and about 240 kilometres west to east, and 160 kilometres north to south. To the east, it bordered the Forostar and Mittalmar regions, but as a coastal land it was surrounded in all other directions by the Great Sea. It also bordered in the coastal south by the small region of Nísimaldar.
Andustar was a fertile region, except along its northern coasts, where the landscape was higher and rockier, and forested by fir trees. Its western shores were broken by two bays, with the northernmost of these being the Bay of Andúnië. On the high land around this bay, the Númenóreans built the port and city of Andúnië, whose Lords of Andúnië were to be the ancestors of the Kings of Arnor and Kings of Gondor descended from.