"The ministry is heavily reliant on beneficiaries' honesty to disclose circumstances so that correct benefits can be paid, and paid to those who are entitled to them," Ms Reihana said.
Kopua's lawyer Frank Minehan said Kopua's partner had been convicted of $5000 benefit fraud and Kopua was "taking on the burden of her partner's culpability, as well as clearly her own".
Mr Minehan said his client "will spend the rest of her life paying back the substantial amount of money that was taken from the taxman".
"If there is an explanation my client was misguided through wanting to provide a safety cushion for her children."
Judge Peter Hobbs said Kopua was granted a domestic purposes benefit in 1995 and agreed to immediately notify the ministry of a change in her living situation.
However, from 2002 to 2008, and 2009 to 2011, Kopua had been in a de facto relationship.
She had not informed the ministry and had in that time submitted eight forms stating that she was single, resulting in a total overpayment of more than $91,000.
There was no need to order reparation, Judge Hobbs said, because the ministry "uses its powers under the Social Security Act to recover this money from you over time". He said the Ministry of Social Development "is, of course, reliant of beneficiaries being honest" in their declarations.
Kopua's actions were "effectively an abuse of trust and a fraud on all taxpayers".
"Unfortunately it also gives beneficiaries in general a bad name when incidents like this occur and are no doubt reported in the press."
Judge Hobbs said Kopua had pleaded guilty and "spared the State the time and expense of a defended hearing".
However, she could not receive full credit because her guilty plea came at a later stage and following an earlier not-guilty plea.
The judge sentenced Kopua to eight months' community detention and 150 hours' community work.
