The New South Wales Government has moved to allay fears that the Hawkesbury River Railway Bridge, north of Sydney, is structurally unsound and putting thousands of lives at risk.

The NSW Opposition cited documents obtained under Freedom of Information laws, which it said showed vital repairs to Pier 2 were recommended in 2013.

The Opposition said the report into the bridge, that connects Sydney's freight and rail commuters to the Central Coast and the Hunter, said repairs should take place within a year.

It said an underwater inspection report showed exposed steel reinforcement due to large amounts of concrete missing and the remaining concrete being soft and crumbly to the touch.

Opposition Leader Luke Foley said more than 600 train services carrying 11,000 passengers used the bridge every day.

"Their safety must be paramount," Mr Foley said.

"This is a disaster waiting to happen: the bridge is quite literally crumbling at its foundations while the Baird Government does nothing."

In response to the claims, NSW Transport Minister Andrew Constance said he had instructed his department to ask the Office of Transport Safety to review the situation.

Mr Constance said he had also called for an independent engineer to inspect the bridge as soon as possible.

"Transport New South Wales says that there isn't any risk to public safety, but I want to be further satisfied with that," he said.

Mr Constance said the work had not been done because of a change in the scope of the project due to a variation in the depth of the river bed.

"As a result of the scope change there's been a change in the cost of doing the project," he said.

Mr Constance said a new tender had to be issued because more extensive repair work was needed and that had delayed the renovations.