Fitch Ratings expects the "Big Four" Australian banks to withstand an expected low-growth and rising unemployment environment over the current year due to significant buffers, the credit ratings agency said in a statement Tuesday.

The strong business profiles and solid financial metrics of National Australia Bank (ASX:NAB), ANZ Group Holdings (ASX:ANZ, NZE:ANZ), Westpac Banking (ASX:WBC, NZE:WBC), and Commonwealth Bank of Australia (ASX:CBA) underpin their long-term issuer default rating of "AA-."

The outlook on Commonwealth Bank of Australia's rating was revised to positive, while the other three banks retain their stable outlooks. The revision reflects the outperformance of the lender's earning metrics. It also revised the funding and liquidity score of the four banks to "a+" from "a."

Collateral positions are strong across the banks on the back of strengthened mortgage underwriting. The lenders' asset quality is expected to weaken modestly, but impaired loans are forecast to peak this year. Their strong liquidity management continues to offset some of the risks from their reliance on offshore wholesale funding sources.

The Australian government is likely to support these banks in times of financial strain due to their systemic importance, Fitch said.