Bellingen resident and independent NSW MP Jeremy Buckingham floated the idea on Twitter, asking the Tesla boss and the Australian Mike Cannon-Brookes how much it would cost to build a 50km tunnel to connect Sydney to the state's west.

"About $15M/km for a two way high speed transit, so probably around $750M plus maybe $50M/station," Mr Musk - who has more than 24 million followers -replied on Wednesday night.

If quoted in US dollars, that would equate to about $1 billion Australian.

Mr Cannon-Brookes, co-founder of Atlassian, said it "sounds like a bargain for Sydney".

Mr Buckingham has been pushing for the project since the 1990s, when he was a councillor in Orange in regional NSW.

He believes the tunnel, which would allow vehicles to be transported on platforms at high speeds, would help ease traffic congestion in Sydney and open up the state's west.

"For about the price of knocking down one stadium, Elon Musk says he can build a transport loop for NSW that would be the envy of the world," he said in a statement on Thursday.

"Autonomous, electric vehicles that generate zero pollution are the future of transport. It would be madness not to further investigate what could be an infrastructure and planning game changer for Sydney and our regions."

Political interest in the big tunnel was welcomed by the NSW Farmers Association, which in August last year published a report arguing the merits of a sub-three hour journey between Sydney and the Central West.

"Good on Jeremy Buckingham for tweeting Elon Musk about our proposed tunnel," NSW Farmers Association president James Jackson said in a statement.

"It's time the NSW government made it a priority to look beyond the Great Dividing Range and see the enormous economic opportunity the tunnel would have for regional NSW and western Sydney."

Australian Associated Press