The Australian Building and Construction Commission is understood to be examining the case, while construction union officials have declined to comment.

In a statement, the commission said it was aware of allegations that workers on a Canberra building site "may have been forced to sign up to a union", but could not comment in relation to the status of any investigations.

“The ABCC stands ready to advise and assist any building industry participant and encourages any worker who has been the subject of pressure to join or not join a union to contact the ABCC,” a spokesman said.

Master Builders Association chief executive Michael Hopkins said the organisation had also been made aware of allegations that employees "were forced to join a union before being allowed to continue work on a local construction site".

"It is appropriate that this matter be referred to the ABCC,” Mr Hopkins said.

The Canberra Times understands the workers spoke little English, and did not understand the agreements they had signed.

Under Australia’s industrial relations laws, all employees have the right to join or not join a union.

Both unions and bosses are breaking the law if they put pressure on workers to make a decision about joining, not joining or leaving a union.

It’s also illegal to take or threaten adverse action against a person for being or not being a member of a union or employer association, for not engaging in industrial action or for choosing to be represented, or to not be represented, by a union.

The allegations come as construction companies have to comply with new rules to tender for ACT government jobs.

The new Secure Local Jobs Code requires businesses to be audited and certified if they want territory-funded work in the security, cleaning, construction and traffic sectors.

Contractors must give the Secure Local Jobs Registrar addresses, working hours and contact details before the job begins to allow unions to exercise their right of entry.

Bosses must also provide new employees union membership forms, and allow union representatives access to their internet service, photocopiers and tea rooms if they want to maintain certification.

Unions had long advocated for the territory government to use its purchasing power to enforce ethical labour standards, off the back of cases where migrant workers had allegedly been exploited by their employers.

In one prominent case, a Canberra school cleaning company was found to have breached the Fair Work Act when  it underpaid migrant workers and signed them up to agreements they did not understand.

CFMEU ACT assistant secretary Zach Smith sits on a new council advising the minister about the operation of the code, along with other union officials, industry representatives and public servants.

However the ACT opposition has repeatedly criticised the new procurement laws, saying they pushed the pendulum too far in the favour of unions.

It is not known if it was an ACT government job site, however the construction company in question has had about $63 million of territory-funded work since July 2015. Calls and emails to the company were not returned.

Do you know more? Email katie.burgess@canberratimes.com.au