Some of Australia's biggest building firms are implicated in improper dealings with subcontractors and union officials on construction and mining sites across the country.

On Friday, building giant Brookfield Multiplex suspended a Construction, Forestry, Mining and Energy Union delegate from a Melbourne central business district project amid allegations of favours and kickbacks on the site involving at least two subcontractors. In other developments, a major mining project run by Fortescue Metals in Western Australia has been infiltrated by a subcontractor linked to the Hells Angels.

A secret Victorian government-commissioned inquiry into the involvement of organised crime in the building sector is understood to have found that a small number of senior employees working for Thiess on the state's desalination plant engaged in improper dealings to farm out subcontracts to favoured firms.

The union delegate at the Brookfield Multiplex site in Melbourne is the third CFMEU official to have been recently forced from their job in connection with allegations improper dealings.

It is understood a senior site manager allowed a union delegate to influence which subcontractors won work. The delegate and his associates are understood to have received in return various inducements from these subcontractors.