The Electoral Commission of South Australia has uncovered more than 600 ballot papers that were not counted in the recent state election, including dozens from the closely contested seat of Narungga on the Yorke Peninsula.

Narungga has been declared for One Nation's Chantelle Thomas, who won the seat by a margin of just 58 votes.

The electoral commission has ordered a further count in Narungga since the discovery.

It says the ballots include 77 unopened absent ordinary ballot papers and four declaration ballot papers that were returned in the neighbouring district of Stuart.