Transport for NSW says it is happy to provide the information but at a cost.

Stop the River Street bridge group spokesperson Karina McLachlain says she applied to get information on how the RMS came to its conclusions through the Government Information Public Access (GIPA) process, only to be told getting the information would cost more than $500 to obtain.

"How can an ordinary person afford to spend that kind of money," she said.

"Especially since the only reason I submitted a GIPA was that Dubbo MP Dugald Saunders told me he didn't know the answers to my questions.

"He advised me he forwarded a letter I sent to him onto the RMS, who have the information that I requested, and he asked them to provide me with the data.

"Therefore, I should have been provided with this information without me having to apply for it with a GIPA."

The Daily Liberal has seen correspondence to Ms McLachlain from the Transport for NSW information access manager that outlines why she should pay for the information.

"Under the GIPA Act, we may charge $30 per hour and the application fee you have provided counts as a payment toward the first hour of any processing charges payable," the manager said.

"Based on an initial search by the relevant business areas for the information that you have requested, I have decided that processing charges for dealing with your application will be required.

"An estimate of total processing charges for dealing with your application is, so far, it has taken 0.5 hours to deal with your application.

"I estimate that it will take at least a further 16.5 hours to search, collect and collate the requested information and to prepare a decision. At this stage, I estimate that it is likely to cost $510 to deal with your application - calculated at $30 per hour."

Before Transport for NSW would process Ms McLachlain's request, it asked her to pay 50 per cent of the money upfront.

Ms McLachlain said it was unreasonable to charge so much money for information that should be freely available to the public.

"This data should be freely available to the public, and my taxes funded the gathering of it in the first place," she said.

"These type of exorbitant charges seem designed to keep information away from the public so that the government can operate without scrutiny.

"I believe that the initial $30 fee that I paid along with the GIPA request is reasonable and I should not have to pay an extra $480 for this information."