At least seven of the investors are believed to be New Zealand-based and Warren, who is living in Whangārei, has been running XM Global Limited for about two years.
One New Zealand woman the Northern Advocate spoke to said her family invested more than $250,000 with Warren after a friend recommended him.
She said the friend had initially had some returns from Warren, which is why he recommended him, and she initially invested $25,000 as a test. When she withdrew $5000 of the investment very easily, she says, this gave her confidence that the scheme was legitimate.
But that turned into a false dawn and she believes her family is now well out of pocket.
''It all seemed very professional and well done, and he [Warren] was very convincing. He said he had safeguards in place to prevent any significant losses that only allowed say 10 per cent of any funds to be invested at one time, so that gave even more confidence.''
Nick Mayer, from NZ Private Investigators, is working with about 45 overseas-based investors who claimed they were ripped off by Warren.
Mayer said Warren is South African and came into the country several years ago.
''David Warren received money from the investors to invest on their behalf using a trading company called XM Trading based in Belize,'' Mayer said.
However, later attempts by the investors to withdraw their money were unsuccessful.
He said neither David Warren nor XM Global Limited are listed on the Financial Service Providers Register nor does Warren hold a Financial Advice Provider Licence.
Mayer said multiple complaints from people Warren has been working with have been sent to the FMA, SFO and Immigration NZ over his alleged actions.
He said he had gone to Warren's listed address in Whangārei and spoke to a woman there, who said Warren no longer lived there.
Mayer said he had tried to contact Warren numerous times, but he had not returned any of his messages or calls.
The Northern Advocate has also attempted to contact Warren, including visiting the Whangārei home listed as his last known address and via phone. The home is empty and no contact details have been left.