Prime Minister Malcolm Turnbull slammed what he said was the banks' culture of greed as political pressure mounted for a royal commission into the scandal-plagued industry.

In a stinging rebuke at a lunch to celebrate Westpac's bicentenary, Mr Turnbull demanded banks broaden their culture away from being motivated only by profit and high salaries, and for honest employees to be rewarded, not punished, for blowing the whistle on crooked colleagues.