SYDNEY (AFP) - Australian construction giant Leighton Holdings, which operates in more than 20 countries, was on Thursday at the centre of a corruption scandal with allegations of bribery among senior executives.

A six-month investigation by Fairfax Media, which obtained hundreds of confidential company documents, exposed what it said were "plans to pay alleged multi-million dollar kickbacks in Iraq, Indonesia, Malaysia and elsewhere, along with other serious corporate misconduct".

In one case, it alleged former chief executive Wal King approved a A$42 million (S$49 million) bribe to a firm in Monaco nominated by Iraqi officials who gave Leighton an A$750 million oil pipeline contract.

Fairfax cited a memo written on Nov 23, 2010 by then acting chief executive David Stewart in which Leighton International managing director David Savage had revealed he and King knew of the massive kickback "I asked did Wal K approve this? And he said 'yes'," the memo reportedly said.

Leighton is a US$7 billion (S$8.76 billion) company active around the world in the telecommunications, engineering and infrastructure, building and property, mining and resources, and environmental services industries.

In a statement to the Australian Stock Exchange, Leighton said it "takes these accusations seriously and is deeply concerned about the suggestions of impropriety", but added that the allegations were "exceptional instances".

It said that in 2011, Leighton voluntarily reported to Australian police a possible breach of its code of ethics relating to accusations of bribery in Iraq, and this was still under investigation.

The statement added that a senior executive was dismissed in 2012 and "over recent years, Leighton Holdings has continued to strengthen and improve its corporate governance and risk management processes".

"The directors of Leighton's subsidiary companies and of Leighton Holdings are aware of their responsibilities and have at all times executed their duties with the appropriate care and diligence, and in the best interests of each relevant company," it added.