Prime Minister Malcolm Turnbull is confident about the accuracy of Treasury calculations his government's new bank levy will raise $1.5 billion annually.
One of the five banks subject to the levy has cast doubt on the size of the annual windfall.
The 0.06 per cent levy is forecast to raise $6.2 billion over the next four years.
Westpac, Australia's second largest retail bank, has advised the stock exchange and shareholders the levy will cost it $260 million after tax annually.
"No company can simply 'absorb' a new tax, so consideration is being given to how we will manage this significant impost on the bank," Westpac said.
Asked by journalists if he was confident in raising the $1.5 billion between the big four and investment bank Macquarie, Mr Turnbull said: "We've relied on the forecasts from Treasury."
He again justified implementing the impost, saying it was widely recognised the big banks do have an advantage over smaller banks.
Finance Minister Mathias Cormann earlier rejected calls to extend the levy to include foreign banks.
Senator Cormann insists there are no major foreign banks in Australia following calls from crossbench senator Nick Xenophon to include them in the policy.
"A feature of our major bank levy policy design is to level the playing field for smaller banks operating here in Australia, to helping them compete with the major banks," he told reporters in Canberra.
