The state electorate of Finniss has an approximate area of 5,900 km².

It is comprised of 70 suburbs including American Beach, American River, Antechamber Bay, Back Valley, Bald Hills, Ballast Head, Baudin Beach, Bay Of Shoals, Birchmore, Brown Beach, Brownlow Ki, Cape Borda, Cape Jervis, Carrickalinga, Cuttlefish Bay, Cygnet River, De Mole River, Deep Creek, Delamere, D'estrees Bay, Dudley East, Dudley West, Emu Bay, Hay Flat and Hayborough plus parts of another 7 suburbs.

It is comprised of the localities of Borda, Cassini, Dudley, Duncan, Encounter Bay, Flinders Chase, Gosse, Haines, Karatta, Macgillivray, Mcdonald, Menzies, Myponga, Newland, Ritchie, Seddon, Vivonne Bay, Waitpinga and Yankalilla plus parts of Goolwa, Kuitpo and Nangkita.

It is comprised of the councils of Kangaroo Island, Victor Harbor City and Yankalilla District plus parts of Alexandrina.

It is surrounded by the state electorates of Hammond, Heysen, Kaurna and Mawson.

It is within the federal electorate of Mayo.