Beneficiaries have been underpaid for the past 18 years - and the Government is now seeking to change the law to avoid paying for the mistake.
Certain beneficiaries have been underpaid by a total of one day each since 1998, beneficiary advocate Kay Brereton said.
An amendment to legislation currently before Parliament would mean such underpayments would be wiped.
"For the past 18 years Work and Income's computer system has been set-up to grant a person's benefit from the day after their stand-down ends," Ms Brereton said.
"However, the law required that the benefit be started the day the stand-down ends - which is one day earlier."