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 (NAB) and Commonwealth Bank face questions over their international business credentials after a British company received the money despite a drastic share price drop over its accounting methods, and acquisitions that attracted media attention.

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. The British company 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''.

In March last year, the share price of Healthcare Locums dropped 30 per cent after it changed the way it accounted for revenue. Months later, the shares dropped a further 25 per cent following weak results.