The Ministry of Social Development says it's taking longer to complete prosecutions for benefit fraud, as fraud cases become more complex.
In a reply to an Official Information Act request by Radio New Zealand, the ministry has investigated 14,937 people for possible benefit fraud in the 2008/09 financial year.
The number of prosecutions completed by the ministry is down, from 1001 last year to 735 this year.
It says it's taking longer to complete investigations, because of the changed nature of the types of frauds being pursued.
The ministry says cases have become more complex, are taking longer to investigate and often require more extensive use of its information gathering powers.
It also says more defendants are electing to plead not guilty, which further adds to the work required to finalise a case for trial.
The ministry says it takes its responsibility for the administration of the more than $12 billion of benefit and superannuation payments a year very seriously.
It says it successfully recovered $700,000 of fraudulently obtained money last financial year.
An Auckland lawyer, Jeremy Bioletti, says some prosecutions are fiscally driven, aimed at getting a person off a benefit.
He says he's personally defended people who have faced serious benefit fraud allegations, only to have the claims fall away in court because they couldn't be substantiated.