The NSW government has been spruiking "unreasonable" estimates of the positive effects of its electricity privatisation plan for the state's economy, a highly critical independent review has found.

The state government released on Thursday a report by ACIL Allen consulting, which was commissioned to review previous research on the projected economic benefits of its plans to sell off the state's electricity assets and spend the proceeds on new infrastructure.

Research, by consultants Deloitte Access Economics, said that the plans would increase NSW's Gross State Product by 3.6 percentage points, or more than $300 billion over two decades.

The growth figure featured prominently in the government's sales pitch for privatisation during the March election campaign. The government also requested another financial adviser on the power sale, UBS, make use of Deloitte's research.

But the damning review found the report sought to put the government's plans in the best possible light and presented a much larger benefit by not factoring inflation into some of its headline conclusions.

The real impact of the government's plans on GSP, the review found, appeared much more modest when presented as a yearly figure.

"That is another way of reporting […] it would increase the annual growth rate of NSW GSP by 0.06 percentage points," the report found. "This seems like rather a modest effect.

"[Deloitte Access Economics] reports its results in ways that maximise the extent to which it results can give the impression that the effects […] on the NSW economy are favourable."

Michael Daley, NSW Labor's Treasury spokesman, said it was the third report to suggest the economic impact of the power sale would be negative or negligible.

"Deloitte has bent over backwards," he said. "The question is whether they were heavied by Mike Baird's staff into reaching these favourable conclusions in the same way that UBS was".

During the election campaign, UBS retracted a research note that suggested that the power sale plan was "bad for the budget". Mr Baird's office called the bank three times before the note was retracted, but UBS has denied the calls influenced its retraction.

But a spokesman for Deloitte Access Economics rejected that its report was not independent and said disagreements over its methodology were not significant enough to bring its broader conclusions into questions.

"We disagree with the many of the review's findings and believe these are not supported by ACIL Allen's analysis," the spokesman said. "In some cases, we believe the findings are incorrect.

"[But] the review finds that our modelling approach is consistent with contemporary practice and that our conclusions are consistent with our modelling results."

ACIL Allen's review found that Deloitte's assumptions of gains to NSW's economy from greater employment and population were "unreasonably high".

Gains from decreased time spent in traffic would also at least partially be offset by a projected increase in immigration to NSW from people seeking to take advantage of better infrastructure, the ACIL Allen report found.

The NSW Treasurer, Gladys Berejiklian, who had promised to release the review as part of a deal to get the legislation enabling the power sell-off through parliament, said the policy still had unambiguous economic positives.

"It is unsurprising that there would be different approaches to measuring the economic gains [of privatisation]," Ms Berejiklian said. "But there is widespread agreement that our infrastructure plan will lift productivity, boost the economy and provide [more] services".

NSW recently sold transmission firm Transgrid for $10.3 billion and is currently running a sale process for distribution firm Ausgrid.