The Serious Fraud Office is investigating the finance company Clegg and Co.
The company went into receivership in October last year owing $15 million to about 500 investors.
SFO director Grant Liddell says the investigation concerns the amounts of money the company was lending to related parties.
Finance companies operate with a trust deed which limits the mount that can be lent to related parties.
The trust deed for Clegg and Co imposed a 5% limit on lending. The SFO says at the time it went into receivership it appears three times that amount was lent to related parties.
Clegg and Co is the sixth finance company under investigation by the SFO.
Mr Liddell says the investigations are complex and likely to take a substantial amount of time.