National Australia Bank said that four staff suspended earlier in January lost it A$360m (US$277m; �153m) in unauthorised foreign currency trading.

The figure is some way below the worst fears of investors, but still leaves the bank needing to explain how the four escaped notice for so long.

NAB, the police and regulators are still investigating the affair.

Questions

The first whiff of the scandal came on 13 January, when NAB had to admit that it had found unauthorised foreign currency transactions on its books.

Four staff, it emerged, had been betting the Australian and New Zealand dollars would fall against the US currency.

When the greenback's slide persisted, it is alleged, they slipped extra trading past management to try to cover their error.

It was a colleague who finally spotted the situation, rather than managers - and one of the traders, 32-year-old David Bullen, has claimed that the bank had agreed a breach of risk limits for more than a year.

The news scared investors, for whom memories of the 1990s collapse of Barings Bank thanks to rogue trading in derivatives remain raw.

NAB's tribulations are nowhere near as large, and the bank says it has identified and closed the loopholes in its procedures that allowed the trading to occur.

But the acknowledged scale of the trades is still a concern, and is double the A$180m NAB originally estimated.