Former deputy prime minister Barnaby Joyce says Australia's banks have abused the support of the government and questioned whether they should be broken up to avoid conflicts of interest.

Admitting his previous opposition to a royal commission into the scandal-plagued industry was "naive" and wrong, Mr Joyce said the banks had "blatantly taken the support of government" as justification to do what they like.

"They should consider whether they should be in the financial planning business after the evidence delivered. It appears from evidence to be too much of a conflict of interest and a temptation for them," he said in a statement to Fairfax Media.

"I have apologised for being so naive to have swallowed their line of 'we are good corporate citizens'. Maybe they should now as well."