“We are in mourning, so far we have 49 dead,” Fedia Castro said on local television, adding that people often travel by truck in this region, because of a shortage of buses.
The victims include 12 children, she added.
The truck tumbled into an abyss in a remote area near the town of Suyucuyo and was carrying revellers.
Castro said an investigation was underway to determine if the driver — who was also killed along with his wife and children — was drunk at the time of the accident.
Deadly crashes are commonplace in Peru, where poorly maintained roads zig-zag up and down the towering Andes.
Worsening the problem, drivers are often inexperienced and buses are known to break down frequently.
In the last week, two other deadly crashes in southern Peru took 19 and 10 lives, respectively.
More than 4,000 people were killed in traffic accidents in 2012, according to official statistics.