Social housing fraud in Northern Ireland 'significant'
As many as 2,400 properties in the social housing stock could be occupied fraudulently, the Northern Ireland Audit Office has said.
A report suggested the problem was "significant" and that "dedicated strategies" were required to tackle the issue.
Between them, the Housing Executive and about 30 local housing associations manage 123,000 properties.
Tenancy fraud is the use of social housing by someone not entitled to it.
The fraud can include giving false information on an application, or obtaining the property and selling the keys to someone else for a one-off payment.
Tenants can also move out and give the property over to a friend or family member, allowing them to "queue jump" the waiting list of 40,000 people.
The Audit Office said there was also a cost to the public purse, given that in 2012 the Northern Ireland Housing Executive spent more than £10m providing temporary accommodation to those classed as homeless.
Auditor General Kieran Donnelly said: "Public funded housing occupied by individuals not entitled to be there is illegal, immoral and unacceptable.
"Such fraud deprives those families most in need of a decent home."
Until recently, the Housing Executive did not have a dedicated tenancy fraud strategy and relied on tip-offs and the vigilance of staff.
The Audit Office made a number of recommendations, like establishing a fraud team between the Housing Executive and housing associations.
The Housing Executive may also improve photographic records of tenants.
In a statement, the Housing Executive said it "welcomes the publication of the report in tenancy fraud by the Northern Ireland Audit Office".
"The Housing Executive will now work with the Department of Social Development and housing associations to ensure that the recommendations are implemented and incorporated in our approach to tackling tenancy fraud."