Z Energy spokesman Jonathan Hill said the company had sold its 1559sq m section on the corner of Tay St and Gordon Rd on November 21.
However, Mr Hill would not reveal the price paid or identify the buyer.
''I will not tell you the identity of the buyer, as that's not fair on them, nor is it our position to tell,'' Mr Hill said in an email.
Mr Hill placed an interview request on behalf of the Taieri Times with the buyer on the day of the sale but the buyer failed to respond before deadline.
Dean Collins, of Colliers International, who marketed the site, did not return messages from the Taieri Times.
The new ratepayer and land ownership information had not been given to the Dunedin City Council and Land Information New Zealand, respectively, before deadline.
The Mosgiel community also remained in the dark.
Mosgiel Business Association chairman Clinton Chambers said he had asked association members if they knew who the buyer was but his inquiries failed to shed any light on the mystery.
''Everyone wants to know.''
Mosgiel-Taieri Community Board member Martin Dillon said the buyer should identify themselves.
''Why do people have to be so secretive? It has to come out in the wash.''
He had heard a rumour the land had been bought to build a Pizza Hut restaurant.
Geraldine Oldham, a spokeswoman of Restaurant Brands, which operates Pizza Hut, KFC, Starbucks and Carl's Jnr in New Zealand, said the fast food company was ''not looking at anything specific in Mosgiel at the moment so, no, this [rumour] is not true.''
