Elections Nova Scotia has fined the governing NDP $10,000 for accepting an illegal campaign donation from a trade union, and has referred the case to police.

The agency said the NDP took more than $5,000 from a single donor and failed to return $45,000 from eight unions and one union affiliate, though it "knew or should have known" the money was all from the Mainland Building and Construction Trades Council.

After the release of the report Thursday, party secretary Ed Wark resigned.

Premier Darrell Dexter referred questions to the president of the provincial party.

"Obviously, I am deeply disappointed," he said Thursday. "When this matter arose during the campaign, I gave direct, clear, specific instructions with respect to the return of the contributions. I don't administer the party."

Party president Peggy Mahon said Wark wrote cheques to the unions, but told them not to cash them until after the chief electoral officer had completed her investigation.

"Darrell was not aware this had occurred. He believed this money had been returned. We all believed the money had been returned," Mahon said.

"Let me be clear. All funds in question have been returned to the original donors with interest. The party gained no financial benefit from these funds."

Mahon said the party only learned it still had the $45,000 in late January when notified by the chief electoral officer. She said Wark had apologized for his "serious error in judgment."

In a release, chief electoral officer Christine McCulloch said the issue was serious enough to merit the maximum fine allowed under the Members and Public Employees Disclosure Act, which governs political contributions.

McCulloch said she referred her report to Halifax Regional Police to investigate as it sees fit, since the act doesn't allow her to look into the unions involved.

The Progressive Conservative and Liberal parties called for an investigation last June after the money was reportedly returned to the unions.

On Thursday, Liberal Leader Stephen McNeil said the NDP had "clearly lied to Nova Scotians."

Political parties in Nova Scotia are not allowed to accept more than $5,000 in a year from a single donor.

With files from The Canadian Press