The post included images of the pastry, which was spotted with blue mould.
The spokeswoman said the sale of the mouldy item was a case of "human error". A casual worker sold an old display doughnut to a customer rather than a fresh one, she said.
"We always want to sell fresh products - the customer shouldn't have been given the sample cake."
She was concerned the store's reputation would be ruined over one mistake.
A member of Auckland Council's Food & Health Licensing team inspected Marcel's Bakery after the complaint, which was lodged on Saturday.
Team leader Etienne Dyssel said it appeared the mouldy item had been "unintentionally sold".
"It appears the operator has procedures in place to deal with old stock but on this occasion, staff did not follow their own procedures and unintentionally sold the mouldy item."
Dyssel said the operator had been instructed to retrain their staff and make systematic improvements, like labelling the food so old stock was not sold to the public.
Scores of people had shared and commented on the post.
The bakery spokeswoman said she hadn't responded to the post, or any of the comments that followed, as she worried it would be "fighting fire with fire".