STAFF error at the Social Security Agency is resulting in around £15 million a year being lost through benefit overpayments.
In total £45m was written off due to staff mistakes between 2013 and 2015.
The amount however has fallen from £21m in 2013 to £18m last year, and represents less than half of all money lost.
Fraud by benefit claimants has cost the taxpayer almost £70m in the three year period, while customer error amounted to £23m.
The figures were released by Communities Minister Paul Givan in response to a written question from SDLP North Belfast MLA Nichola Mallon.
She said: "These staggering losses through customer fraud and staff error will not instil much confidence and demonstrates the failings that remain in the system.
"The minister is very vocal on his plans to tackle benefit fraud, we need to hear exactly what he is doing to tackle staff error."
Mr Givan said that the total amount lost remained around one per cent of all expenditure.
The minister said: "The amounts lost through staff error are consistently lower than that lost through customer fraud and error.....This was a remarkable achievement against what is a highly complex benefit system, and represents a strong staff performance - recognised as such by the Audit Office and the Independent Joint Standards Committee.
"Nonetheless, small margins of fraud or error can result in significant sums...it is therefore crucial the performance evidenced to date continues," he said "to protect the public purse".
In July, The Irish News reported that benefit fraud investigations have cost more to carry out than they have recouped.
Almost £4.6m was recovered from investigations over two years, while £5.3m was paid out in salaries.
