The Role of Credit Rating Agencies in Australia’s Residential Mortgage‑Backed Securities Market After the 2025 Housing Correction

RMBS and the Housing Downturn

Australia’s residential mortgage‑backed securities market, worth over A$300 billion, functions as a critical funding conduit for non‑bank lenders and smaller banks. Credit rating agencies assign ratings to tranches of RMBS deals, from AAA to unrated, based on the quality of underlying home loans, loan‑to‑value ratios and structural protections. When the Australian housing market corrected in 2025, with national dwelling prices declining 5.4 percent peak‑to‑trough, rating agencies responded by adjusting their assumptions about borrower defaults and loss severity. Those adjustments directly influence the volume and cost of mortgage funding available to households, making RMBS ratings a barometer of broader financial system health.

Rating Migration in 2025‑2026

S&P and Moody’s updated their Australian RMBS criteria in late 2025, raising the weighted‑average foreclosure frequency assumption for non‑conforming loans by 15 percent. This led to a small but notable migration of junior tranches from A‑ to BBB, though senior AAA tranches remained largely insulated. By mid‑2026, credit enhancement levels for new prime RMBS deals had increased by an average of 40 basis points, meaning issuers needed to pledge more collateral for the same rating. While the shift added modest pressure on origination costs, it also demonstrated that credit rating agencies were able to reflect emerging risks without triggering panic, preserving investor confidence in Australia’s securitisation market.

RBA’s Securitisation Dataset Insights

The Reserve Bank of Australia’s March 2026 Financial Stability Review provided granular data: the share of AAA‑rated tranches in prime RMBS rose to 91 percent of total issuance, up from 88 percent two years earlier, as weaker loans were carved out or credit‑enhanced. Mortgage arrears 90‑plus days past due, concentrated in the lowest‑rated tranches, remained benign at 0.8 percent. This resilience allowed non‑bank lenders to continue accessing wholesale funding markets, maintaining competitive pressure on the major banks. Without the disciplining and signalling role of credit ratings, the post‑correction RMBS landscape would have been far more opaque and costly, potentially choking off credit to first‑home buyers and small businesses reliant on non‑bank finance.