A group of senior road tunnelling experts have rubbished entrepreneur Elon Musk's claim he can build a 50-kilometre loop through the Blue Mountains for $1 billion, with one describing it as "Alice in Wonderland" talk.

Key points: Experts say Mr Musk's tunnel idea could cost as much as $50 billion

Experts say Mr Musk's tunnel idea could cost as much as $50 billion The NSW Government has not received a proposal but would take it seriously if one came form the tech billionaire

The NSW Government has not received a proposal but would take it seriously if one came form the tech billionaire The tweet sparked debate about transport visions for Australia

Mr Musk sparked debate yesterday after replying to a speculative tweet from NSW MP Jeremy Buckingham on the cost of a tunnel, which has since been liked by nearly 25,000 people.

The Boring Company founder predicted the tunnel, running through the mountains from near Lithgow to outside of Penrith, would cost $US15 million a kilometre.

But experts yesterday questioned the tech billionaire's maths, with a chorus of senior tunnelling engineers — including one advising fellow billionaire Richard Branson's Virgin hyperloop tunnel project — dismissing the cost synopsis provided by Mr Musk.

Engineer and hydrogeologist Philip Pells — one of the senior personnel behind the Sydney's harbour and Eastern Distributor tunnels — said Mr Musk's prediction was "totally out of the ball park".

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"He's out by at least a factor of 10 and up to a factor of 50," Dr Pells, a tunnelling industry expert for 40 years, said.

"The difficulties of this sort of project are substantial.

"I'm not trying to knock the guy, but he's not going to be able to build a tunnel through the Blue Mountains for $1 billion.

He's out of his league … [he's] dreaming — it's Alice in Wonderland stuff."

Mr Musk's proposal in the Blue Mountains is based on a similar tunnel system is being tested in Los Angeles where cars are lowered into the tunnel on a lift before being moved through it on platforms at high speed.

Another is under construction in Chicago.

The proposal comes at a time when Sydney is transitioning through a phase of high population growth, with a number of multi-billion-dollar tunnel projects such as the $14 billion Western Harbour tunnel as well as the WestConnex and NorthConnex tunnels being built to ease the congestion.

An example of the tunnel Mr Musk proposed for Sydney ( ABC News )

NSW Transport Minister Andrew Constance called Mr Musk one of "the great minds on the planet", but said the proposal "isn't a reality at this time".

Transport for NSW conducted basic costings on the LA-style project yesterday, predicting a $6 billion price tag for a road and rail tunnel — and $3 billion for just a road tunnel — with "ventilation" and "emergency egress" adding extra costs.

Other key issues included geotechnical conditions and integration with the wider transport network, it said.

"I absolutely love his vision," Mr Constance said.

"That's [the] kind of innovation in years to come which could form part of the transport network.

"If there's any unsolicited proposals we will absolutely take it very seriously, but at the moment what is being looked at from his perspective isn't a reality at this time."

One engineer predicted the tunnel could cost as much as $50 billion, not $1 billion. ( Supplied: Jeremy Buckingham )

For world-renowned tunnel engineer Arnold Dix, Mr Musk's "thought bubble" was the type of conversation starter that attracted global attention to the opportunities underground.

"He's managed to popularise concepts and extreme engineering that no one was talking about previously," said Professor Dix, who is advising Richard Branson's Hyperloop project.

"On that note I say it's not necessarily a bad thing. And if he's prepared to sign up to $1 billion price tag — like he did with the South Australian battery project — then I say sign him up."

But according to Professor Dix it was just that — a thought bubble.

"He was probably just sitting on his jet with nothing to do," he said.