A new $6 billion tax on banks tipped to be revealed in Tuesday night's budget would likely be passed straight onto customers, says a major investment bank.

The Australian Financial Review reported on Tuesday afternoon that a so-called "Tobin Tax" was set to be unveiled in the budget.

The tax would be levied on financial transactions and the prospect of its implementation helped send bank stocks into a tailspin on Tuesday as more than $13 billion was wiped off the value of the banks.

UBS analyst Jonathan Mott said although the structure remained unclear it was likely to slice about 5 per cent off the banking sectors profits.