The Greens are confident they have sufficient support to have the Senate call for a royal commission into banking sector misconduct.
A similar move a year ago failed when Labor voted with the coalition.
The Greens will make a second attempt on Monday when parliament returns for a special three-week sitting.
"I'm confident the Senate can send a powerful message to the government that we do need a royal commission," Greens senator Peter Whish-Wilson told ABC radio on Friday.
The move is likely to test the resolve of Nationals senator John Williams, who crossed the floor to vote with the Greens and crossbench last year.
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Senator Williams is one of a handful of government MPs backing a royal commission.
Labor has changed its position, pledging to establish a royal commission if its wins government.
Opposition Leader Bill Shorten would not say whether Labor would back the Greens.
"The Greens don't tell Labor what to do," he told reporters in Sydney.
