A senior official has accepted responsibility for a failure to properly scrutinise flaws in the Renewable Heat Incentive.
The Department of Enterprise, Trade and Investment (Deti) set up the botched energy initiative in 2012, with energy regulator Ofgem tasked with running it.
Elaine Dolan was the department's head auditor at the time and she told the RHI Inquiry that Ofgem's arrangements should have been scrutinised more effectively.
She added that under-resourcing in the department had been an issue and her team had been too "stretched" to give the matter more attention.
She said at first she didn't realise "how novel or complex" the RHI scheme really was.
Her team offered to provide advice and guidance in Deti's energy division, with the option for it to approach the audit team if any problems were then encountered.
She said this was a "real opportunity missed" to give more detailed assistance at the start of such an unknown and complex project.
She said this in particular would have been of great help in establishing the arrangements between Ofgem and Deti.
"That's one of the areas where I think we could have provided value in getting the scheme set up right," she said.
In 2014 an outside accountancy firm was hired to assist the department.
It accepted assurances from officials in Deti's energy division that Ofgem had the overall responsibility for running RHI. Despite this, Ms Dolan said the audit gave her the impression that Deti did not have enough of its own monitoring arrangements in place.
She said the audit should have established factors such as a risk register, project plan and a senior person responsible for the scheme.
A lack of professional scepticism, she said, had been applied to assurances over Ofgem's responsibility to manage the scheme.
Ms Dolan said that in 2014 her primary focus was not RHI, but identifying risks in telecoms schemes for Deti.
She took a career break late in the year, but her plan had been to turn her attention towards energy schemes such as RHI in 2015.
RHI Inquiry chair Sir Patrick Coghlin said: "Unfortunately that was a bit late."
Panel member Dame Una O'Brien thanked Ms Dolan for "taking responsibility for those areas where the internal audit service did not perform its role adequately".
She added: "They're perhaps words we hear too infrequently so far in this inquiry.
"I appreciate that it takes a lot given the nature of your work for you to have come and said that, and thank you for being so frank with the inquiry."
During her evidence, Ms Dolan also said that a businesswoman who told Deti officials the scheme was being abused should have been listened to.
Janette O'Hagan had raised her concerns on several occasions only to be ignored, but Ms Dolan said this should have prompted an investigation.