(Adds comment from Amec executive)
SAO PAULO, March 11 (Reuters) - Shareholders, management and regulators need to reassess the business model that drove Brazil's state-controlled oil company Petróleo Brasileiro SA to lose hundreds of billions of dollars in market value in nearly six years, minority shareholder association Amec said on Wednesday.
Amec said that Petrobras, as the company is known, needs a "radical shift" in management, while respecting the federal government's status as a majority shareholder.
The group added that a corruption investigation of Petrobras, which has triggered a 60 percent decline in shares since last September, should be carried out in a "thorough and independent way."
If Petrobras improves corporate governance and transparency, it can avoid the need for a capital increase, which some analysts see as imminent, Amec Vice President Andre Gordon said, adding that was his personal view and not Amec's.
Petrobras investors may have lost as much as $330 billion due to the graft scandal and unwanted government meddling, Amec calculated, based on the opportunity cost of an average investment in the oil industry.
"All the agents involved - the controlling shareholder, management, investors, financial intermediaries and regulators - must consider their obligations toward the company that was once Brazil's largest," the group said in a statement.
"Management must reconcile returns compatible with risk and the legitimate public interest that led to the creation of the company," it added.
Petrobras' 10-member board has two seats for independent members and one for the company's workers.
The remaining seven board members represent the federal government and state companies and funds, which hold about 48 percent of outstanding shares. (Reporting by Guillermo Parra-Bernal Editing by W Simon)