Sydney’s economy will get an almost $20 billion boost if government invested in affordable housing in the inner city, slashing commute times and increasing job opportunities for the city’s workers, researchers say.

Unaffordability is steadily spreading “like an oil stain” to middle and outer suburbs, according to SGS Economics and Planning partner and index author Ellen Witte. According to the latest rental affordability index, even a typical Melbourne household pocketing almost $90,000 in income would struggle to find an affordable family home to rent within 15 kilometres of the city centre. The index – which tracks rental affordability using census data on household incomes and rental bond information lodged with government departments – found the median rental household in greater Melbourne earned $88,400 each year.