Health insurers owe the HSE almost €300m for the treatment of their subscribers.
The finding comes from the latest report by the Comptroller and Auditor General, which says the taxpayer is meeting the funding gap until payments are made.
The Health service also wrote off €4.6m in debt owed in private patient charges in 2014.
The Comptroller and Auditor General's annual report for 2014 identifies issues across a number of areas in the public sector.
In terms of the HSE, it notes that health insurance companies owed €290m for private patient income at the end of last year.
That is made up of €172m owed to HSE hospitals and €118m to State-funded voluntary hospitals.
It says that despite efforts by HSE management to get consultants to sign off on claims in a more timely manner, the situation has disimproved.
It notes that the Exchequer is effectively meeting the funding gap connected to the outstanding debt until payment is made.
The HSE said it will work with the Department and insurers to improve the charging regime.