He contacted the Otago Daily Times yesterday to say he intended to plead guilty when he appeared in Dunedin District Court next Thursday.
In 2005, he defended a drink-driving charge after he drove with a blood-alcohol level of 116mg. The limit is 80mg.
He had not believed he was over the limit and tried unsuccessfully to defend the charge on technical grounds.
He was convicted and fined $750, with $130 court costs, and disqualified from driving for six months.
Acklin said yesterday he would carry on with his council duties and last night planned to inform council colleagues he had not already told.
Mayor Dave Cull said he became aware of the charge earlier this week.
Under the Local Government Act, the councillor's actions had no implications on his tenure, he said.
"He's remorseful about it. It's regrettable when anybody does something like that."
The Local Government Act requires councillors to resign only if convicted of a crime that carries a possible prison sentence of two years or more.