The Ulster Unionist Party has been fined for failing to declare £12,000 (€13,500) of its spend on last year's Assembly election.
The party was handed the £3,000 penalty by the Electoral Commission for lodging an inaccurate return.
Bob Posner, from the Electoral Commission, said: "Experienced political parties such as the UUP are expected to be able to follow the rules regarding spending at elections."
The UUP, which has paid the fine, has commissioned auditors to review the party's accounting processes and to recommend improvements.
"The Electoral Commission is pleased that the UUP is reviewing its procedures and hopes that this will ensure that a mistake of this nature is not repeated," said Mr Posner.
"It is important that the spending of political parties and campaigners at elections and referendums is fully transparent and the rules are correctly followed."
