Australia's five largest banks are prepared to launch a mining tax-style ad campaign to fight the Turnbull government's $6.2 billion new tax, with Bankers' Association chief Anna Bligh accusing the government of playing "fast and loose" with Australia's financial system.

Ms Bligh said Treasury officials were unable to provide key details of how the new tax would work and that the big banks were concerned they may end up paying much more than $6.2 billion over four years.

Speaking after banking representatives met Treasury officials in Canberra on Thursday to discuss the new tax, Ms Bligh suggested the measure may have been "cooked up" in the Treasurer's office and put in to the budget as recently as last week.

But Treasurer Scott Morrison hit back at suggestions Treasury officials had not yet worked through how the $6.2 billion would be collected and said the banks had turned up and asked: "Could we pay less tax?"