Labor believes it has the Senate support to force the government to hold a royal commission into banks but would need coalition MPs to cross the floor in the lower house.
Opposition consumer affairs spokesman Sam Dastyari is quietly confident, saying there had been a lot of support for the inquiry from Nationals and a few Liberal MPs.
"Right now if we were to move a motion through the Australian Senate calling on the government to have a royal commission it would sail through," he told ABC TV on Sunday.
"Whether or not it would pass the House of Representatives would really depend on whether any of the conservatives are going to cross the floor."
But he said Labor would make tactical decisions about whether to try and force the inquiry closer to parliament's sitting at the end of August.
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Opposition Leader Bill Shorten wrote to the prime minister on Friday asking him to "walk the final step" on the path to more transparency and accountability in financial services by supporting Labor's push for a royal commission.
He says Malcolm Turnbull's plan for bank executives to appear before a parliamentary committee was likely to disappoint because it lacked the powers and credibility of a royal commission.
"Unless the Australian banks are compelled to produce documents well in advance of those hearings - as they would in a Royal Commission - they will be little more than a forum for the banks to justify their own behaviour and culture," Mr Shorten wrote.
