The Serious Fraud Office is investigating a potential fraud which could involve more than $250 million.
The revelation came from departing SFO director Adam Feeley, who said investigators had started working on the case in the past few weeks.
He said the investigation work was at an early stage but "we don't like what we see so far''.
Mr Feeley, who yesterday announced he was leaving the SFO to run Queenstown Lakes District Council, said the suspected fraud was "upwards'' of $250m.
"The scale of the numbers involved are very large.''
If the case turned out to be fraud, it would rank among the country's largest white collar crimes.
Other significant cases overseen by Mr Feeley during his three-year tenure include the SFO investigation into South Canterbury Finance owner Allan Hubbard which led to charges to the value of $1.7 billion.
The case never went ahead after Mr Hubbard died in a car accident. Other significant cases included the $100m case against DataSouth owner Gavin Bennett and the country's largest employee fraud of $17m by ASB Bank manager Stephen Versalko.