SYDNEY — The home wasn’t perfect. The two-bedroom townhouse lacked parking and a backyard. Its proximity to a major road and a train station made for noisy evenings.

But like many young Australians, Georgia Blackie felt she needed to buy it or rent for the rest of her life.

She lives in Melbourne, one of the world’s wildest and most expensive real estate markets. The values of dwellings there have risen by more than 50 percent in the last six years alone. In Sydney, Australia’s other big real estate capital, the increase has been even higher. The two cities are among the world’s least affordable for housing, according to one survey, worse than famously pricey places like New York and London. Mortgage debt puts Australian households among the world’s biggest borrowers.

Lately it has cooled off, though, and people like Ms. Blackie may pay the price.

She and her partner closed on the townhouse last August for 720,000 Australian dollars, or about $533,000. But since the market’s peak in November, neighborhood home values have slipped about 6 percent.