The lawsuit filed with the Court of Queen's Bench in Saint John alleges that Grant Thornton was in breach of its duties to the province in an inspection of the financial books of the Atcon group of companies.
The former Liberal government of then premier Shawn Graham provided the loan guarantees to the companies in 2009.
The statement of claim alleges the government approved the loan guarantees as a result of financial reports from Grant Thornton.
"But for the Grant Thornton opinions and representations, the province would not have sustained a loss in excess of $50 million," the document asserts.
It says the accounting firm "failed to exercise the care, diligence, and skill of an auditor of reasonable competence and prudence."
None of the allegations contained in the statement of claim have been proven in court.
Atcon, based in Miramichi, went bankrupt in April 2010.
Grant Thornton LLP, Grant Thornton International and a chartered accountant who works for the company are named as the defendants. They have not filed a statement of defence and the accountant named in the statement of claim could not be reached for comment.
Norm Raynard, managing partner in New Brunswick for Grant Thornton, said in an emailed statement that the company would not offer specific comments on the lawsuit because they are still reviewing the court documents.
"We will vigorously defend ourselves against this action," he wrote.
"Our initial reaction is that this timing has much to do with the political calendar in the province."
The next provincial election is set for Sept. 22.
Attorney General Hugh Flemming said the timing is not political.
"This is not a time schedule which in any way was influenced by the government and it is not a political issue," he said Tuesday.
He said the government had no choice but to take the matter to court because of the $50 million that was spent.
"The government owes a duty to the people of New Brunswick to do what they can to recover this," Flemming said.