Macquarie Bank shares plunged the most in nearly six years today, wiping more than a tenth off the Australian bank's market capitalisation after it warned one of its funds could fall foul of the spreading sub-prime malaise.

Macquarie shares dropped 10.7% to A$73.7 by today's trading close from A$82.5 the previous day as investors reacted to the bank's announcement that the value of its Fortress fund could have fallen by up to a quarter last month.