Westpac is being sued for alleged insider trading, amid claims the bank unfairly profited from a $12 billion interest rate swap deal.
Financial regulator the Australian Securities and Investments Commission on Wednesday said it was taking the bank to the Federal Court over the deal with an AustralianSuper consortium.
ASIC alleged that on October 20, 2016, Westpac traders knew the bank would be chosen by the consortium for the swap.
The consortium had, about 90 minutes earlier, agreed to buy electricity provider Ausgrid from the NSW government.
ASIC alleged Westpac traders used the inside information to buy and sell interest rate derivative products on the market to better position the bank for the swap.
Interest rate swaps allow two groups to exchange interest rate payments, often fixed for variable, over a set period.
Westpac said it took the allegations seriously and was considering its position.
Shares in the bank were higher by 0.42 per cent to $26.10 at 1115 AEST.