National Australia Bank (ASX:NAB) is set to issue AU$1 billion of subordinated, fixed-to-floating rate notes due 2041 on Tuesday, according to a same-day filing with the Australian bourse.

The callable notes can convert into ordinary shares of National Australia Bank if the Australian Prudential Regulation Authority deems it necessary on the grounds that the company would otherwise become non-viable, per the filing.

The company's shares fell 2% in recent Tuesday trade.