The commissioner said a care plan was never established and lack of communication between doctors meant the infection was always treated in isolation, recurring to the point of amputation.
In a statement, Te Whatu Ora Whanganui, formerly the Whanganui District Health Board, said it acknowledged the findings from the report.
"The failure to have a co-ordinated care plan in place to manage this patient's care has resulted in the amputation of a leg.
"We take responsibility for this situation. Our thoughts are with all who have been impacted by this."
Te Whatu Ora Whanganui said it wanted to reassure all patients that a great deal of work had been completed since the incident, the statement said.
A high-risk foot clinic had been established which was focused on education, awareness, assessment, diagnosis and care plan development, initiation of treatment, follow up and onward referral.
"We will continue to review our practices and processes to ensure patient care and safety is at the heart of everything we do."