The government has grossly overestimated the cost of extending copyright laws in the Trans Pacific Partnership (TPP), by tens of millions of dollars, an Australian economist says.

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The economist, George Barker, told the Foreign Affairs, Defence and Trade Select Committee that the error was made by an external economist.

The economist was hired by the government, and Dr Barker thought the mistake was possibly made by "misplacing a decimal point".

That economist had estimated the cost per annum of the extension as $55 million for the film, books and music industry, Dr Barker said.

But his figures were different.

"If you take one industry that forms part of that estimated cost - music - [the government is saying] that's contributing $17m to that $55m. But that number should be around $77,000," Dr Barker said.

He tried to get hold of the original data but the Ministry of Business, Innovation and Employment (MBIE) had been unable to give it to him, he said.

RNZ News has approached MBIE for comment.