The plan to knock down and replace a 144-year-old bridge in Sydney's northwest is expected to cost double what the NSW government quoted to the public.
The Windsor Bridge replacement project is estimated to cost $101 million according to the Roads and Maritime Services final business case seen by AAP.
In its environmental impact statement in 2012, the RMS said the project would cost $50.4 million.
That amount was again affirmed at a NSW upper house inquiry into the project in April where it was also revealed that $30 million had already been spent on the project - before construction had begun.
The historic two-lane bridge - built in 1874 for horse-drawn vehicles - is slated for demolition with the state government planning to build a replacement bridge 35 metres downstream and a connecting road to Windsor's historical Thompson Square.
The three-lane replacement bridge will improve traffic and safety for motorists says the RMS, which argues parts of the existing bridge are "deteriorating substantially" with the structure reaching "the end of its economic life".
The project, first flagged in 2008, has sparked outrage from local residents who are fighting to keep the original bridge, and are pushing for the RMS to build a bypass nearby instead.
The final business case notes the "significant opposition" from the NSW Heritage Council and the community and concedes the project would "adversely impact" the significance of the state-heritage-listed Thompson Square and the overall values of Windsor.
The RMS says mitigation measures will be in place but admits impacts on heritage will "not be totally mitigated".
A RMS spokesmn said that as with all major projects, costs are subject to change.
"Work on the bridge is scheduled to begin in September 2018," the spokesman said in a statement to AAP.
Local group Community Action for Windsor Bridge (CAWB) says it has "deep concerns" over the project and its budget "overruns".
Construction was due to start in June but was delayed after the RMS discovered historical artefacts nearby.
The upper house inquiry, chaired by Shooters MP Robert Brown, is due to release its final report into the project in August.
A spokesman for NSW Heritage Minister Gabrielle Upton said the project had been approved with several heritage conditions.