Great swathes of the Sydney metropolitan area are now considered beyond the reach of the city's growing number of renters with the closest affordable suburbs for average income earners now revealed as Blacktown and Liverpool.

Australia's first rental affordability index has found Sydney is the most expensive city for renters, with nothing affordable between Newport in the north and La Perouse in the south.

Even suburbs once deemed cheap such as Lidcombe and Lakemba have been pushed into the unaffordable category by spiralling property prices, leaving renters with an odious choice: live in poverty close to the city or move further out and spend hours commuting.

The index found the most affordable area in the greater Sydney region is Black Springs, almost 200km from the central business district.