New Zealand's not alone with housing issues.

The ratings agency Moody's is warning the risks of mortgage defaults in Sydney and Melbourne are growing, as soaring house prices are matched with bigger loans and deteriorating affordability.

Housing affordability in Australia's two biggest cities during the past year has been hit by house prices rising sharply and wages remaining flat.

Sydneysiders living in two-income households are spending 35.1 per cent on mortgage repayments; in Melbourne it's 28.2 per cent.