German company Howe has employed 14 men since mid-June to carry out building work at the Schanzen post office building in the Swiss capital.
The workers have also been sleeping at the construction site, in the basement of an air raid shelter.
As the work is being carried out at the premises of Swiss Post, Unia representatives held a meeting with the state-run company on Friday calling for it only to hire firms that offer proper wages and decent accommodation for their workers.
According to the Neue Zürcher Zeitung, Swiss Post is investigating the allegations, but it has already asked Howe to comply with national wage agreements for construction workers.
Two weeks ago, Unia revealed similar findings concerning 40 workers from Bosnia employed for the construction of a new municipal incineration plant, also in Bern.