Some of the men are working on a new building in the city's south for the federal government, which has pledged to crack down on "rorts" in the 457 visa program.

According to statutory declarations from some of the workers employed by Mitchell-based KP Pro Group, workers have been forced to work unpaid overtime each week and to wait up to six weeks for payment.

But the company has denied exploiting its workers, dismissed one of the complainants as a disgruntled former employee and said its trouble with paying wages was a direct result of two of Canberra's recent high- profile building company collapses.

About 100 union members led by the building union CFMEU converged on one of KP's sites early on Tuesday morning, an apartment construction project on Mort Street in Braddon, in what they have dubbed a "community" protest.

A former employee of the company said that the wages of the Australian nationals hired by KP were "prioritised", with workers from overseas forced to wait up to six weeks for their money.

One Korean declared that he was sponsored by the company last December to work as a painter in Australia on a 457 skilled migration visa despite never having worked in the trade.

The man, Taehyoung Jo, said he was recruited through a Korean language website just after finishing his accounting degree at Macquarie University in Sydney, and worked an average of 55 hours a week while being paid for just 38.

KP Pro Group, which employs more than 30 Koreans, undertakes painting, joinery, paving and tiling for residential and commercial projects throughout Canberra.

John Phillips, the company's former contract manager who has also signed a statuary declaration alleging exploitation, said that the KP employees on the federal government's new Centrelink building in the Tuggeranong suburb of Greenway had been forced to work 14-hour shifts for no overtime pay.

Mr Phillips said he decided to speak out because he was disgusted by the company's treatment of its Korean workers. "The way these guys have been treated has been quite disgusting and they [have just as much] right as anybody to be treated fairly," he said.

"They don't get paid at the same time as the Australian employees,"

Mr Phillips said KP's overseas workforce had recently been forced to wait for several weeks for their wages. "Out of 68, only 25 got paid and the rest didn't get paid for about six weeks," the former manager said.

"They get asked to work from 6.20am until whatever in the afternoon but [in] the joinery section, they might be working there until 10 o'clock at night with half an hour lunch break. If told to go home, they'd say 'we can't, because the boss will be angry'."

KP's director, Jin Ho You, said his company was struggling after the local firms Ply ACT and Urban Contractors went into administration owing KP a combined $1 million. The director conceded that wages had been paid late but said the only prioritising was towards men with dependent families.

"We've been waiting for pay from when Ply happened; there's seven hundred grand owed to us," he said.

"There is outstanding overtime but the boys are aware of this and I'm very happy for you to talk to them about it. They'll get paid, but we don't have any money at the moment.

"We got hit by Urban Contractors, there's a massive debt outstanding there too."

Mr You denied Mr Phillips' accusations, saying his staff did not even start work at 6.20am.