Three Sinn Féin offices in Northern Ireland were mistakenly each paid £10,000 they were not entitled to from an emergency Covid-19 business support scheme.
The payments were made automatically shortly after a small business grant scheme was launched by the Stormont Executive in March.
The money has been repaid to Land and Property Services, which administers rates payments.
It is understood the repayments were made this week, after the issue was highlighted by BBC Radio Ulster's Nolan Show on Monday.
Sinn Féin did not apply for the grants, which were paid to 24,700 small businesses, with 452 recipients later deemed to have been ineligible.
The first 7,000 grants, which included those to the Sinn Féin offices, were paid automatically into the bank accounts of businesses that paid annual rates of up to £15,000 by direct debit.
The guidelines for the scheme stated that offices for MPs and members of the Stormont Assembly were not eligible.
In a statement, Sinn Féin said: "Three Sinn Fein offices received automatic and unsolicited payments of £10,000 under the Small Business Grant Scheme.
"Sinn Fein offices did not qualify and did not apply for the scheme and the monies have been returned to the LPS (Land and Property Services)."
The Department for the Economy, which administers the scheme, said 70 of the 452 businesses who received grants in error have paid them back.
It is understood the total paid in error was more than £4m.
The DUP has criticised Sinn Féin for the delay in repaying the £30,000 paid to it, with the grants understood to have been returned this week.
Gary Middleton, an MLA in Foyle, called on the Stormont Finance minister, Sinn Fein's Conor Murphy, to explain what happened.
"Registration for the Business Support Grant Scheme closed on 20 May. It is unacceptable and unjustifiable that this money is only now being repaid several months later," he said.
"We are still in the dark as to exactly when Sinn Féin representatives found out about this error."
He added: "It is time for Sinn Féin to be straight with the public about its knowledge and response to these issues.
"At a time when many businesses are fighting to survive the integrity of the public purse is of paramount importance."
The DUP, SDLP, Alliance Party and Ulster Unionists have all said none of their offices received payments.
The error may have arisen because Sinn Féin owns its own constituency offices and pays rates on them, whereas other parties lease their offices and the rates are paid by the Northern Ireland Assembly.