Despite the name, many affordable homes are out of reach for low-income households because rents are often set at a proportion of market rate, rather than a household's income level.
The system is failing the households that need it most, the think tank wrote in a report on Thursday.
As well as setting rents above what many low-income earners can afford, broad eligibility criteria mean medium-income households often have access to affordable housing, crowding out those that need it most.
Analysis by CEDA and consultancy group Urbis found only 19 per cent of rentals affordable to a household eligible for social housing actually go to one.
In Sydney and Melbourne, that figure was 18 per cent while in Perth it was one in 10.
"Affordable housing is meant to relieve pressure on the social housing waitlist, but right now it's not reaching the households who need it most," said report co-author Danika Adams.
"If we could move just one in ten households off the waitlist and into affordable housing that actually matches their income, almost 19,000 families would have a safe, secure home today, without building a single new dwelling."
Social housing differs from affordable housing in that it is government subsidised, allocated to people in need rather than open to the market, and rent is usually based on a household's income level.
Greens Senator Barbara Pocock said the lack of a national definition for what constitutes an "affordable" home means property developers have been allowed to cash in on the housing crisis by advertising homes above median market rates.
"Our housing system is cooked," a spokesperson for Senator Pocock said in a statement.
"It's time for the government to get back in the game of directly building, owning and operating public homes like they used to."
Senator Pocock will grill officials from Housing Australia and Treasury at a parliamentary committee hearing into social housing on Thursday.
Social housing advocacy group Everybody's Home, which will also appear at the inquiry, said the share of social housing in Australia fell from roughly six per cent in the 1990s to just four per cent of households in 2025.
"Over the last four decades, Commonwealth support for social housing fell from around $2.5 billion in 1982 to $1.6 billion in 2022, while Australia's population grew from 15.2 million to 26.1 million," Everybody's Home said in a submission.
"On a per-person basis, spending fell from about $164 to $61."
