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.''