Warning signs for NSW mobile speed cameras don't fit in dozens of the cars

Nearly a third of New South Wales' mobile speed camera cars are at risk of being replaced because the signs don't fit in the boot.

A transport estimates hearing revealed 47 out of the 143 mobile speed camera fleet cannot fit the signs in the vehicles, which are set to be rolled out from January 1 next year.

Transport for NSW deputy secretary Tara McCarthy said the government is looking at alternatives for the signs, including lighter weight options.

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"There's a number of vehicles in the current fleet that don't have the capacity to hold the same signs we used previously," she told the hearing.

"My understanding is the vendors have changed the fleet and some vehicles are smaller than previous fleets and the back of the vehicle is fully taken up with cameras, and equipment.

"The signs previously used are quite heavy, they need to be properly mounted, they need to be properly restrained."

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The signs will be required before and after the mobile speed camera car but McCarthy said the 47 cars simply can't fit them inside.

McCarthy revealed the cars will either need to be significantly modified by removing seats or replaced on the taxpayers' dime if a different sign option can't be agreed upon.

"We're looking at a lighter-weight sign that can be collapsed down and folded in half so it takes up less space but we need to test the robustness of the sign," she said.

"We would have concerns if it blew over."

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McCarthy was unable to answer how much it would cost the government, and taxpayers, if the mobile speed camera cars need to be replaced.

She said it would fall under the existing contracts, which cost the government a total of $168 million.