The federal treasurer has urged at least one of Australia's five big banks to break ranks and declare that they will not pass a new levy onto customers, in the hope the others may follow.
Scott Morrison accused Westpac, NAB, ANZ, Commonwealth Bank and Macquarie of "having a lend of people" over the $6.2 billion, four-year tax, and said they could afford to wear the cost.
He told reporters in Brisbane that the first bank to break ranks and absorb the new tax will get a lot of support from Australians.
"And if the other banks don't want to follow them, then I'd be following the bank that is the first to move."
Consumer watchdog chief Rod Sims has assembled a surveillance squad to work out how the banks calculate fees and charges on home mortgages to try and monitor whether the levy is unfairly passed on.
The Australian Competition and Consumer Commission will be given access to confidential bank emails, internal reports and other documents, and also have the power to hold compulsory hearings under oath.
"We don't have the power to stop the banks doing anything but the fact we're looking and have an obligation to report publicly will have an effect on their behaviour," Mr Sims told ABC radio.
But federal Labor leader Bill Shorten said giving extra money to the watchdog to track the levy was a useless gesture because even the ACCC admitted it had no teeth.
He called on Prime Minister Malcolm Turnbull to explain to Australians exactly how much they will pay as a result of the new tax.
"What they need is a banking royal commission and Malcolm Turnbull's too weak to give it to them" Mr Shorten said.
Labor has also accused the government of policy-on-the-run with apparent indecision about whether the levy will be imposed on foreign banks.
It followed comments by the treasurer's assistant minister, Michael Sukkar, that he would not rule in or rule out extending the measure.
"In the next sitting fortnight you will see the legislation that has been well thought through and, as I've said, any good ideas along the way we will look at," he told Sky News.
Meanwhile, Mr Morrison downplayed complaints from the big banks about having to sign a confidentiality agreement over legislation for the levy.
Australian Bankers' Association chief Anna Bligh claims the government is going to extraordinary lengths to keep the tax hidden from those most affected by it.
"A bad tax has now become a secret tax," she said.
But the treasurer said it was not an irregular practice, noting something similar was done for its multinational tax avoidance and diverted profits tax legislation.
"It'll ensure that we're able to work through the issues that we have, which are very minimal, so the legislation can be introduced on time," Mr Morrison said.
"These are sensitive matters."
Labor wants the draft legislation to be made public, with Mr Shorten accusing the prime minister of colluding with the banks about how the levy will work.