Victoria’s proposed Windfall Gain Tax will stifle development and cut new housing supply by adding up to $25,000 per housing lot in areas such as the regions where affordable land is most needed, industry estimates show.

As disparate industry groups united on Monday to oppose the 50 per cent tax that the state government said it would levy on the extra value created from rezoning a piece of land, Pitcher Partners executive director Craig Whatman said the move could halt new development.