Chiselled out of sandstone beneath the heart of Sydney’s CBD, a cathedral-like cavern and tunnels are growing by the day as construction workers dig deeper. The underground labyrinth that is emerging near landmark buildings will become the signature station for the $25 billion Metro West line between the CBD and Parramatta.

After Metro West opens in 2032, Hunter Street will be the busiest station on the underground line for city-bound commuters in the morning peak, when up to 10,000 people an hour are forecast.

For engineers, the station’s construction comes with a twist. Its location at the eastern end of the underground line means a giant tunnel has to be dug so that the eastbound trains can cross to the other side of the railway, before heading back in the opposite direction. Apart from crossover tracks for trains to return to Parramatta, the so-called turnback will be large enough to store a 120-metre-long driverless train.

Twin tunnels almost a kilometre long will also extend under the Domain, passing beneath the heritage-listed State Library. In a move described as “future proofing” the mega project, the excavation of so-called stub tunnels about 45 metres below ground will allow for the rail line to be extended to Sydney’s southeast in coming decades.

Eastern tunnelling project director Bob Nowotny said excavating in a tight space beneath the heart of the CBD presents major engineering challenges.

“From an engineering standpoint it’s an engineer’s dream to be building this cavern because of the scale of it, where it’s located and the engineering that goes in behind what we do,” he said. “We are more sculptors than we are tunnellers – sculpting the rock to fit all the infrastructure in.”

Nowotny, a 35-year veteran of underground projects around the world including road tunnels and railway caverns, said the Hunter Street station was more difficult than others he had worked on because of its CBD location. “You can see the vastness of the operation. There’s a lot of logistics and planning that goes into it,” he said.

Three giant machines known as road headers are scraping away at the sandstone around the clock, carving out the singular cavern about 180 metres long for the station’s two platforms. Most of the other stops on Metro West will be binocular cavern stations, which have separate tunnels for each platform.

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The roof of the main cavern is about 12 metres below Hunter Street, while its floor is about 30 metres deep. The alignment of the main cavern and tunnels follows as closely as possible that of Hunter Street above to avoid office skyscrapers’ deep basements.

Every day, special trucks carry soil excavated from the depths of the cavern and tunnels up a steep, winding road to a site near the surface. Giant ventilation machines blow air into the cavern and tunnels, where workers wear breathing masks. Workers have completed almost a third of the excavation.

Sydney Metro project delivery head Josh Watkin said the station would act as a gateway to the northern end of the CBD when complete, connected by underground pedestrian links to the new Metro City and Southwest station at Martin Place and nearby Wynyard station for Sydney Trains services.

“The size of this is so unreal when you consider that it is happening under people’s feet. It is hard to convey to people what is happening underground – it’s in a league of its own,” he said.

“It will sit at the intersection of a number of lines. It’s not only a terminus destination in its own right but also a major thoroughfare underground for people connecting to all the other lines. In terms of the pedestrian walkways and customer accessibility, it absolutely does have some challenges for us to maximise those flows, so there’s no crowding.”

A train will arrive every four minutes at the station, while another will head in the opposite direction to Parramatta and Westmead.

If an ambitious plan to redevelop Rosehill Racecourse and build an extra station for Metro West on the valuable site fails to be realised, travel times from Hunter Street to Olympic Park will be 15 minutes, and to Parramatta 20 minutes. A trip under Darling Harbour to Pyrmont will take two minutes.

Transport Minister Jo Haylen said the Hunter Street metro station was set to revitalise the commercial heart of Sydney’s CBD. “We’re building Sydney’s second under-harbour railway crossing, which is the final tunnelling piece in this massive public transport project,” she said.

“By connecting our city’s two CBDs from Parramatta to Hunter Street, passengers in between will have access to fast and reliable metro services.”

Two giant boring machines will begin worming their way from the Bays precinct at Rozelle within months towards the Hunter Street station, creating twin 3.5-kilometre rail tunnels. They are due to break through into the western end of the Hunter Street station’s main cavern later next year.

Under the wider plans, a skyscraper up to 51 storeys will be built on the corner of Hunter and George streets above the station’s western entrance, metres from Wynyard station and light rail stops. Another tower of up to 58 storeys will be erected at the corner of Hunter, O’Connell and Bligh streets, near the upmarket Rockpool Bar and Grill.

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Pub and restaurant billionaire Justin Hemmes is part of a consortium competing against a group led by global investment giant Brookfield, which redeveloped Wynyard station, for the right to complete the Hunter Street metro station and build the towers above.

Watkin said the contract for the over-station development was expected to be awarded next year. “We’re working with two shortlisted consortia at the moment, but it’s a work in progress,” he said.

Internal Sydney Metro documents have previously estimated that the Hunter Street station will cost $640 million, making it one of the most expensive to build along the 24-kilometre route of the underground rail line.

Watkin declined to put a figure on the station’s cost, saying it would depend on the delivery timeframe for the Hunter Street terminus and the procurement model for the over-station development.

The tunnelling work is due to be completed late next year, when it will be handed over to other contractors to build and fit out the CBD station.

The average depth of the Metro West tunnels will be 38 metres – about 13 storeys – compared with 35 metres on the Metro City and Southwest line, which is due to open in the middle of this year, and 27 metres on the Metro Northwest railway between Rouse Hill and Chatswood.