Job-hunting agency Invest NI was allowed to "write off" £7m of taxpayers' money in a venture capital scheme.
But private investors who matched the public funding got back more than £13m, Enterprise Minister Arlene Foster disclosed.
Now there are fears a further £10m may also have to be effectively written off in the future.
Jim Allister MLA, who obtained the information in a series of Assembly questions, attacked what he called a "cavalier approach" to the use of public money.
He said it was "monstrous" for private investors to have seen a 100% return while taxpayers got zero.
The £7m of public funds involved in the fund, which operated between 1995 and 2007, was effectively lost because it was 'subordinated', which means it was declared secondary to any return after the private investors had made their profit.
The Department of Enterprise, Trade and Investment (DETI) poured £7m into the project called 'Crescent Capital 1' which it ended up writing off, although the private investors who also invested £7m, got back £13.27m - a return of 7.8% per annum - while the fund managers collected £3.6m in management fees, Assembly answers said.
"The DETI investment was written off. Nothing was returned to DETI," Ms Foster said in an Assembly response. "This subordination of public sector funding was considered necessary in order to secure the required level of private-sector funding," she added.
Mr Allister said he had also noted that Invest NI now intends to hand over another £10m into its recently-announced Development Fund, again on a wholly subordinated basis.
The Traditional Unionist Voice leader predicted: "Again, on past performance, we can expect this to be another write-off of public money, with only private investors ever likely to see a return."
Mr Allister's questions also established that a successor venture-capital project, 'Crescent Capital II', was established in 2004 which also features the jobs agency as a subordinated investor.
Ms Foster confirmed: "Crescent Capital II was set up in March 2004. It is a £22.5m fund and Invest NI is a 33% investor alongside a number of private investors. The initial cost to Invest NI was therefore £7.5m."
There was no further comment from DETI or Invest NI last night.
Invest NI, which has been praised in recent times for exceeding its job creation targets, set up a venture capital scheme called Crescent Capital whose primary focus was with locally-based firms operating in the information technology, life sciences and manufacturing sectors. Crescent Capital itself received £3.6m in management fees over the 12-year period of the initial fund.