TWO of Australia's biggest banks have lost as much as $150 million on a loan gone bad within weeks of it being approved.

National Australia Bank and Commonwealth Bank face questions over their international business credentials after a British company received the money despite a drastic share price drop as a result of its accounting methods and acquisitions it made that attracted media attention.

National Australia Bank and Commonwealth Bank face questions over their international business credentials.

Described as ''quite an extraordinary event'' by one source, the banks have discussed writing off most of the money handed over last December to Healthcare Locums - Britain's biggest provider of casual medical staff. It froze trading on its shares in January, and declared it had to investigate ''accounting irregularities''.

One London source, close to the loan process, said both banks ''have a track record for not really doing particularly thorough diligence when they finance [an international] transaction''.