Cornwall Housing delivers services on behalf of Cornwall Council and was also found to have out-of-date electrical safety certificates for 1,000 homes.
Tenants have been "reassured" that "immediate action" is being taken.
Housing portfolio holder Olly Monk said the council wanted to make sure the company delivered for its customers.
"We are not going to turn around the whole thing overnight but will be working with haste on the electrical safety," he said.
In addition a second report by Pennington looked at compliance with health and safety checks including electrical safety, gas safety checks and asbestos.
It found that Cornwall Housing's data management in relation to these was "poor" and that internal controls for compliance were "weak".
It recommended a "back to basics" approach to all compliance issues for the group that manages 10,500 homes in Cornwall.
'It's a big job'
The Altair report said there was an "adversarial" relationship between the company and the council and claimed the process used to secure and pay for temporary accommodation could have resulted in "significant lost income".
Cornwall Housing's interim managing director Iain Sim, who came into post in March, said the reviews indicated the scale of the challenge in front of him.
He said: "To be honest with you a lot needs to be done.
"There are a lot of challenges in there and a lot of issues in both reports.
"I have been asked to come in and get us back onto the right road and deal with the weaknesses going forward.
"It is a big challenge as there are fundamental issues that we need to address. It is a big job."
He added: "We are contacting all our tenants and leaseholders to reassure them that we are rapidly addressing the findings in the report as the safety, health and wellbeing of our residents is our top priority".
In relation to electrical testing, the report said the company was moving from a 10-year testing cycle to a 5-year cycle but found that 1,073 properties were outside the 10-year inspection cycle.
Altair recommended the company prioritise any outstanding inspections "to reduce potential risk to customers".