The Ministry of Social Development would be given extended powers to seize partners' assets in order to recover costs.
There were expected to be 700 cases of debt splitting a year once the legislation had been passed.
The amendment to the law would relate to partners of people who claimed a Domestic Services Benefits or Sole Parent Support despite being in a "marriage-type" relationship.
The penalty for partners involved in relationship fraud would be a fine of up to $5000 or up to a year in prison.
In addition to the law change, the ministry would introduce tougher rules for beneficiaries who had been dishonest in the past.
Three-quarters of the people who were charged with welfare fraud in the last financial year had previously ripped off the ministry.
These "low-trust" beneficiaries would have more restricted access to self-service transactions and would face more rigorous verification of their personal information.
The new policy would affect around 1000 beneficiaries.
The ministry would also formalise information sharing with related agencies such as ACC, Inland Revenue, Housing New Zealand and the police.