If there is any latent interest in an explicit bank tax in Australia, the Customer Owned Banking Association is taking steps to exploit it.Aware that an interim report into Australia's financial system is just weeks away, COBA has launched a national advertising campaign questioning the cost to the community of Government guarantees to four of Australia's most profitable companies.The campaign, for which COBA has booked spots on television, radio, online and in print, will use four large skyscraper buildings growing bigger and bigger in an animated ad to highlight the reduction of competition and lessening of choice for banking consumers. COBA's chief executive, Louise Petschler, called the Financial System Inquiry "a once in a generation opportunity to put the interests of customers first and produce a more competitive banking market." "We need a better banking system - one that stops delivering unfair advantages to the big four," Petschler said. The COBA TV commercial points to Australia's big four banks, branded by the campaign as 'too big to fail', and therefore likely to be bailed out by the government in a crisis. A report released earlier this year by Morgij Analytics estimated the subsidy to be worth A$2.5 billion, and COBA argues the big four should pay for this guarantee.The campaign might strike a chord in Canberra as the budget bills approach the Senate.The government's awkward politics of the budget is the type of event that promotes reactive tax reform.A senate majority exists to block tax rises favoured by the coalition government.Some will have to be replaced by tax measures that attract that same majority.This thinking must inform a revival of proposals for a specific bank tax, something becoming common in G20 markets and endorsed by the IMF.In July 2013, The Greens pushed for a levy on the assets of big banks. The party proposed a new tax of 0.2 per cent on banks with assets of more than $100 billion.The Greens projected then that the bank super-tax would raise $8.4 billion over three years.The possibility of a tax on Australian banks - on their capital or earnings or both - must be rising in the local political climate.