In a review for the government, Mr Stevens was critical of all grants to home buyers but singled out those going to non-first-time buyers as particularly wasteful.

"It is important to note that at present about half of the grants made by the government do not go to first home buyers: they go to those buying their second or subsequent home, if it is a new dwelling," he wrote in his report to the Premier in May. "There may have been a case for this to assist the building industry during the financial crisis, but the builders don't need that demand stimulus now: what they need is faster processes for converting zoning and development applications into construction and an ability to respond to those segments of the market that will accept smaller, lower-cost dwellings."

Independent economist Saul Eslake said it was possible the scheme had led to "some marginal increase in supply" but "the most significant effect would likely have been to fatten builders' profit margins by something close to the amount of the grant".

Initially unlimited, the scheme was modified in 2014 to exclude foreign buyers and to introduce a cap of one grant per buyer per year.

But a new breakdown of the scheme, obtained by the NSW opposition under a freedom of information request to the NSW Department of Finance, reveals the extent of the double-dipping that occurred.