In what authorities described as the largest gold heist in Canadian history, suspects including an Air Canada employee forged an airway bill to steal the cargo that had arrived in Toronto from Zurich in April 2023.

The cargo contained 6600 gold bars - weighing 400kg worth more than C$20 million ($A22.5 million) - and C$2.5 million in foreign currency, the Peel regional police told reporters at a briefing.

The suspects have been hit with more than 19 charges by the authorities, who recovered one kilogram of gold and approximately C$434,000 in Canadian currency, police said.

The stolen gold may have been melted and reconstituted, making it difficult to track down, police said.

Police also recovered 65 firearms that investigators allege were purchased from the proceeds of the theft.

"We believe that they've melted down the gold and then the profits they got from the gold, they used to help finance the firearms," Detective Sergeant Mike Mavity said.

Five suspects were arrested in Canada and conditionally released.

One suspect, originally from Brampton, Ontario, was arrested in Pennsylvania along with the firearms and remains in custody in the United States.

Canada-wide warrants have been issued for the remaining three suspects, police said.

Following the heist, Miami-based transport and security services provider Brinks sued Air Canada in October, alleging "negligence and carelessness" by the airline.

Brinks had been hired by Swiss bank Raiffeisen Schweiz and precious metals refining company Valcambi to move the valuables from Switzerland to Canada.

Australian Associated Press