The announcement, in October that year, followed a couple of years of the government musing over its options for a duplication of the highway through the pinch-point Blue Mountains village of Blackheath, including a bypass.

The plan grew more ambitious seven months later when it was announced that the tunnel might also burrow under Mount Victoria, meaning it would run 11 kilometres from Little Hartley to Blackheath.

Following changes of government at the state and federal level, the Great Western Highway tunnel has since been paused, cancelled and criticised for being impractical and billions of dollars short of the funding required - but digging continues in various locations on the metropolitan side of the Blue Mountains.

Just a couple of weeks after the October 2020 Blackheath announcement, the almost nine-kilometre Northconnex road tunnel opened to traffic in Sydney's north-western suburbs after tunnelling of about two-and-a-half years that ended in late 2018.

The Northconnex tunnel does have a toll, whereas the NSW Coalition had been adamant the Great Western Highway tunnel would be toll-free.

The Sydney Metro West railway line, meanwhile, will run for 24 kilometres underground and tunnelling for part of that project, from The Bays to Olympic Park, was completed in December 2023.

Tunnel boring machines have since set off from Five Dock and are heading towards Burwood North, averaging 200 metres a week.

In the far west of the city, tunnelling is continuing on the Sydney Metro Western Sydney Airport railway line.

On a much smaller scale, three tunnels are planned as part of the 14-kilometre Pacific Highway bypass of Coffs Harbour, the longest of which will be 410 metres.

An environmental impact statement (EIS) for the Great Western Highway tunnel, released in early 2023, said tunnel boring machines had been identified "as the preferred tunnelling methodology for the mainline tunnels as they can excavate at a faster rate than roadheaders and are able to install precast structural, waterproof tunnel lining progressively".

"The selection of the TBM [tunnel boring machine] tunnel construction method would ensure that tunnel construction is sensitive to the unique environmental and cultural surroundings of the Blue Mountains," the EIS said.

"It would also provide a value for money and sustainable construction method, given the precast tunnel lining would add to the tunnel's longevity."

Using two TBMs launched from Little Hartley was identified as the preferred option "to minimise impacts at the eastern [Blackheath] portion of the project", as well as reducing the demand for labour and construction materials during the course of the tunnel construction (as opposed to having four TBMs running).

There has been speculation that the Great Western Highway tunnel might have cost up to $11 billion.

In 2022, the NSW Government's independent body Infrastructure NSW recommended a number of "megaprojects", including the highway tunnel, be diverted to smaller projects that will provide "high returns and faster paybacks with less budget and delivery risks".