Gilles Cloutier was arrested and questioned by provincial police on Sept. 2 in connection with 15 allegations of perjury.
No charges have been laid.
Cloutier testified before the province's corruption commission in 2013.
His bombshell testimony touched on everything from cooked books to straw man fundraising, but it primarily focused on elections he was brought in to engineer outside of the city of Montreal.
During his time in the witness box, a Parti Québécois lawyer accused Cloutier of lying.
Cloutier says he only told the commission the truth.
However during his testimony, he admitted he had lied about one thing: he was not the owner of a home used by engineering firm Roche to woo politicians.
That lie as well as several others led to his arrest, according to Radio-Canada.
The police investigation into the perjury allegations was only revealed during Cloutier's testimony at another trial.
That fraud case involves the former mayor of Boisbriand and employees at some Quebec engineering firms.
While he was being questioned, Cloutier reportedly told police key members of the Charbonneau Commission came to him after his testimony to reassure him about the lie he already admitted to his in testimony.
According to court documents, Cloutier alleged that lead counsel Sonia Lebel told him not to worry about his testimony.
Lebel denies this.
The defence is now arguing Cloutier is no longer credible or reliable.