A party spokesman says the New Democrats are willing to pay about $2,000 for sending surveys on health care to the people in the riding of Dartmouth-South.
Mark Laventure says NDP caucus mailouts were approved by the Speaker on March 13 for 15 ridings, including Dartmouth-South.
However, the party delayed sending the flyers to the riding until mid-April after the Liberal member representing the riding died on March 16.
Laventure said the party failed to take into consideration the rules prohibiting a publicly funded mailout during a byelection period.
"We had a conference call with Elections Nova Scotia (Thursday) and confirmed that ... there had been a breach," he said.
"The rule is there and it should be followed. It was just something we hadn't considered when we sent it out."
Elections Nova Scotia was already investigating the Liberal caucus, which sent 33,000 flyers into the vacant ridings of Cape Breton Centre, Sydney-Whitney Pier and Dartmouth South during the third week of May.
Premier Stephen McNeil has said his party wasn't clear on the rules and on Thursday he said it appears other parties weren't either, despite changes to the Elections Act that were implemented in 2011.
Under the rules a byelection campaign begins when the seat becomes vacant. In a general election the campaign begins when the writ is dropped.
Parties are banned from using government money for partisan materials during an election campaign, though the funding is permitted at other times.
"I think everyone was under the impression that what happens at a general election applied to a byelection," said McNeil.
McNeil has said the Liberals will pay the $7,000 cost of their mailouts.
Both the Liberals and NDP say they will abide by whatever ruling is made by Elections Nova Scotia.
Dana Doiron, a spokesman for Elections Nova Scotia, said the agency expects to announce Friday what actions it will take on the mailouts.