A consortium led by the toll road operator Transurban purchased a 51 per cent share in the motorway for $9.3 billion in 2018.

That money was used to fund the M4-M5 Link, NSW Treasurer Dominic Perrottet said, and the funds raised from the sale of the other 49 per cent will receive the same treatment.

"Proceeds from any potential transaction will be invested into the NSW Generations Fund (NGF) and allow us to continue to build world-class infrastructure such as the Metro West train line from Sydney to Parramatta," Mr Perrottet said in a statement on Friday.

But NSW Shadow Treasurer Walt Secord said it was unwise to sell the asset amid the economic downturn prompted by the COVID-19 pandemic.

"It will be difficult for the Berejiklian Government to get proper value for money in the current economic climate," Mr Secord said in a statement.

"Who tries to sell off revenue earning sources into a depressed market during the worst economic crisis seen in generations?"

Mr Secord is demanding the treasurer release the scoping study and treasury advice that backs his decision to sell the motorway stake.

The first stage of the $16.8 billion road project opened to traffic in July 2019, with the second opening in July this year.

When the final stage of the WestConnex is completed in 2023, the 33km motorway will allow motorists to travel from Penrith to the city without traffic lights.

The motorway will be operated by Transurban, as agreed during the 2018 sale of the majority share.

Australian Associated Press