Australia's major banks receive an implicit subsidy worth between $1.9 billion and $3.7 billion from the market's assumption that taxpayers would come to the rescue in a crisis, the Reserve Bank of Australia has estimated.

The funding advantage that the big four banks gain over smaller rivals from being "too big to fail" peaked in the global financial crisis, but was still present when an internal report was prepared in 2015, documents published under Freedom of Information laws showed on Wednesday.

The finding from the RBA comes as regulators around the world look to limit the risk that taxpayers will be called on bail out banks, by forcing lenders to issue more "bail-in" bonds that would force creditors to wear bigger losses in a crisis.

Due to their critical role in the economy, investors assume big banks would be rescued by government in the event of a financial crisis.