An Auckland gold trader is now in liquidation and is being investigated by the Serious Fraud Office.
Grace Holdings, which trades as Bullion Buyer, is under scrutiny after the SFO was contacted by a number of investors following media reports last week.
In one case, an investor was told he would not be repaid $340,000 invested in leverage options with Bullion Buyer.
The SFO says it moved swiftly to assess the complaints and secure the company's financial records.
Financial markets general manager Simon McArley says up to 70 people with millions of dollars of investments are affected.
He says the agency does not have an exact figure, but knows that more than $3 million has been invested, and it's unsure how much of that has been accounted for.
Mr McArley says the SFO is working with the Registrar of Companies to protect any funds for the benefit of creditors.
Meanwhile, Grace Holdings blames its failure on one of its former traders Elijah Gus Geldman.
In a statement, the company says Mr Geldman is in custody in the United States and it has laid a complaint with the Serious Fraud Office over his actions.
Grace Holdings says it called in liquidator Grant Reynolds after it failed to arrange funding from an overseas party to secure its investors.