Investigators weren't obliged to tell beneficiaries their statements couldn't be challenged in court, which often led to prosecutions - sometimes unfairly, he said.
A new law that came into effect will hold both partners in a relationship equally accountable for benefit fraud.
The nature of today's modern relationship meant it wasn't as black and white as being married or not, and many beneficiaries didn't have a solid understanding of the difference, he said. "The whole criteria has been blurred ... Yes, they might share a house, but if a woman is subject to domestic violence where the male partner controls her to what extent can she be said to be getting emotional and financial support from that person?"
Rotorua district councillor Merepeka Raukawa-Tait said those being prosecuted for benefit and relationship fraud were usually women.
"[Women] are easier to go after. I'm pleased that this law change now has the potential to level the playing field ... "
However, women's safety during the prosecution process was a concern, and not something likely to be considered by the ministry.
"They will concentrate on prosecuting and the women will be at risk. There needs to be further discussion around the likely impacts of this law change and what measures need to be in place so that we don't end up transferring costs to another social service area [like] mental health."
Nationally, 906 beneficiaries were successfully prosecuted - establishing fraud debt worth more than $32.6 million.
Under the Social Security Act, anyone who makes a false statement to claim benefit money illegally can face a maximum penalty of 12 months in prison and a $5000 fine.
More serious cases of benefit fraud can come under the Crimes Act, in which case offenders can spend up to seven years in jail.
The ministry's deputy chief executive of student, seniors and integrity services, Iona Holsted, said all fraud debts owed to the Ministry of Justice were actively followed up, often with the help of police asset recovery teams.
The ministry had a range of powers to recoup the debt including stopping offenders from leaving the country, she said.
However, repayment was flexible and done on a case by case basis, she said.
"These repayment arrangements are reviewed and adjusted if a client's circumstances change.
"Sometimes however there are circumstances when clients are unable to repay their debt, for example if they are in prison or unable to be located."
The total balance of fraud debt owed to the Crown is currently $128 million.