The law gave a “perverse” incentive to install smaller wind turbines, the SDLP’s energy spokeswoman said.
The method of obtaining a higher proportion of renewable sources was “badly flawed”, Sinead McLaughlin added.
She said the Audit Office report made worrying reading.
“We see that investors have made in some cases excessive profits, the cost of which has been borne by electricity prices that were too high.
“This has been a complaint of business for several years.”
She said the objective of achieving a higher proportion of renewable sources for electricity was laudable but the mechanism for doing this was badly flawed.
“In particular, legislation created a perverse incentive to build smaller wind turbines, rather than the more cost-effective larger wind farms.
“Bearing in mind the failings around the Renewable Heat Incentive (RHI), it seems that government in Northern Ireland has difficulty in getting to grips with the concept of perverse incentives.”
She said the audit office report also revealed other failings.
“Financial support was paid in respect of plant that did not have planning permission to operate and for plant that generated electricity that did not actually go into the electricity grid.
“These failings were the result of poor attention to detail in the drafting of legislation – the auditor refers to the legislation being ‘vague and non-specific’.
“This is just not good enough.
“We should expect more from both the Northern Ireland Civil Service and from ministers.
“The Assembly and Executive must ensure that these types of failings do not recur.”