Public housing has fallen from 12.4 per cent of Canberra's housing stock to 7.1 per cent in the last 27 years, with applicants now waiting on average nearly 1000 days for a place.
The ACT's ageing public housing flats are being sold off and replaced with up to 1288 new homes across Canberra.
However more than 1700 people are on the waiting list for public housing in the territory.
Applicants can wait anywhere between 274 days for priority housing, through to 983 days for standard applications.
While there are affordable housing targets for new suburbs, only 3.5 per cent of the greenfield sites scheduled for release this financial year will be for public housing.
Greens crossbencher Caroline Le Couteur will on Wednesday file a motion in the ACT Legislative Assembly, calling on the ACT government to ensure public housing stocks fall no lower.
Ms Le Couteur said the safety net of public housing had "gotten a bit too small, it's not fitting all of us".
"We all know someone who’s either currently in public housing or in need of public housing, there are so many people not very far away from needing it themselves," Ms Le Couteur said.
"These are the sorts of people who are one problem away from finding themselves in housing stress.
"Their car breaks down. They lose their job. They have to find the $400 for their teeth and they can’t pay the rent the next week.
"There are a lot of people in that situation in Canberra who haven’t got really safe, secure housing and having a decent backstop would make it easier for all of us."
Ms Le Couteur said while 99 per cent of the ACT's public housing went to people in greatest need, against a national average of 74 per cent, this also meant Housing ACT had little own-source revenue to invest in more properties.
Her motion will ask that the territory government also implement policies to grow the supply of affordable rental housing targeted at low to moderate income households, where rent is set at a discount to market.
Canberra rents are among the highest in the nation, putting extra pressure on the city's public housing stock.
Last month, Fairfax Media revealed former tenants of the Stuart Flats were squatting in neighbours' living rooms and laundries after being evicted with nowhere else to go.