Australia's big four banks are at risk of another a credit agency downgrade.
Standard & Poor's in May cut the ratings of 23 small Australian lenders, but retained those of ANZ, Commonwealth Bank, National Australia Bank and Westpac, reflecting expectations of support from the federal government in the event of a housing crash.
"There is a chance we could downgrade the big four banks," Gavin Gunning, S&P senior director of financial institutions ratings, told a briefing on Tuesday.
If Australia's sovereign rating, which is already on a negative outlook, came under pressure, that would have an impact on bank ratings.
Also, if there was a shift away from government support for the banks, as seen in the US and Western Europe.
"If this trend also takes hold in Australia that also could also impact the Australian major ratings," Mr Gunning said.
Moody's Investors Service downgraded 12 Australian banks, including the big four, on Monday citing elevated risks within the household sector heightening the sensitivity of Australian banks' credit profiles to an adverse shock.
While it does not anticipate a sharp housing downturn, the risk represented by increased household sector indebtedness became a material consideration in the context of the very high ratings assigned to Australian banks, the agency said.
The downgrade suggests bank funding will be slightly more expensive when raising money overseas, a factor that has forced higher independent interest rate increases in the past.
The downgrade came as a double blow to the big four banks after parliament passed legislation imposing a new levy on them.
The levy, announced in the May budget, will be imposed on the big four and Macquarie from July 1 and is expected to raise $1.6 billion in the first year.
Treasurer Scott Morrison has rejected a recommendation from a government-dominated Senate committee to review the levy after two years, as requested by the banks.
The committee also said the legislation should be amended to allow the treasurer to suspend the levy in cases where banks are in extreme financial hardship.
"There is no need to do any of those things," Mr Morrison told ABC radio on Tuesday.
"The bank levy has been legislated as I said it would be."
The committee also said the Treasury should better explain why foreign banks were excluded.
Finance Minister Mathias Cormann told the Senate the levy would apply to foreign banks if they were ever to meet a liabilities threshold but, at present, none fits the "major bank" category.
The indexed threshold starts at $100 billion.
Treasury conceded to a Senate hearing last week its modelling assumed some "pass through" to customers and shareholders from the levy.
Bank shares fell on Tuesday, contributing to an overall decline on the Australian stock exchange.