The big four banks have been lifting rates on investor and interest-only loans after the Australian Prudential Regulation Authority moved to tighten lending in those areas amid concerns about heightened risk in the housing market.

The regulator wrote to all banks last month, outlining new requirements for banks to reduce interest-only lending to 30 per cent of total mortgage lending. Currently the banks sit at around 40 per cent.

The Australian Securities and Investments Commission is also conducting surveillance in the interest-only lending sector to identify lenders and mortgage brokers who are recommending high numbers of more expensive interest-only loans.

Interest-only lending allows borrowers to pay back only the interest on a loan over a period of time, usually up to five years. They are considered higher risk because the repayments rise sharply once the interest-only period ends.

CBA said home loan customers who already had a fixed rate loan with the bank would not be affected by the changes.