People are being invited next month for the first public tour inside New Zealand's largest infrastructure project, Auckland's $4.4 billion City Rail Link.
The Government/Auckland Council joint business has just announced plans for the rare glimpse inside the mega-project on Sunday, November 17.
It will offer free tickets to the first 10,000 people who register.
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"Aucklanders will have the opportunity next month to walk a section of the 3.45km-long tunnels," CRL said, adding that tours would start at the Britomart end in lower Queen Street.
Tickets will be available from Wednesday, 6 November.
"This will be a rare opportunity for people to enjoy a brief snapshot of New Zealand's largest infrastructure project being built right under the city - we have a lot to celebrate," said CRL chief executive Dr Sean Sweeney.
"It's a great chance for us to say, 'thank you' to people for the support they are giving the project, and for us to show off some of the outstanding engineering behind a project that will re-shape the way Aucklanders travel," he said.
People will be able to walk under lower Queen St, the $1b almost-completed 40-level Commercial Bay development and into lower Albert St, a return distance of 600m that will take about 30 minutes, a statement said.
Space in the tunnels is confined. Access for those who use wheelchairs or mobility is limited. There will be no access for bikes, prams, pushchairs, scooters and skateboards.
Numbers will be restricted to 10,000. Entry will be by ticket, which will only be available on-line from November 6. Tickets will be free and restricted to six per person. Walking the tunnels will run between from 8 am until last entry at 5 pm.
There will be 10 entry sessions during the day. To keep people safe underground, each session will be restricted to 1000 people.
When tickets become available, people will need to nominate the time of their visit.
The tunnels are unsuitable for people with a fear of being in a confined space, and there are low levels of lighting underground, CRL said. Wear sensible, flat shoes.
Entry and exit from the tunnels will be via the Britomart station and the day will be supported by Auckland Transport, KiwiRail, and rail-operator TransDev.
Details are still being finalised and people will be told of the full programme and how to get their online tickets soon, CRL said.
"Our priority is to keep everyone safe in confined spaces underground and to make sure they go home with an experience that they'll remember for the rest of their lives," Sweeney said.
Open days at the 2.4km Waterview Connection twin-tunnel motorway project drew tens of thousands of people. In 2017 just before it opened, around 42,000 people booked an open day visit.
Last week, CRL signalled the first open day on social media. The project is not due to be finished until 2024.
CRL asked people how keen they would be. "Me!" wrote an enthusiastic Auckland councillor Cathy Casey.
People remembered the Victoria Park motorway tunnel project and its open day, including a sit-down black-tie dinner before the cars flowed.
Families expressed enthusiasm to see inside CRL, one vowing to travel from Wellington on the Northern Explorer train to go underground.
"Great idea but you will need to allow plenty of opportunity and allocated ticketing, as there will be huge interest, as gauged by the Waterview tunnel experience," said one follower, encouraging the business to avoid disappointing those interested.
Others asked for money to be charged, saying that could support charities. "This is a once-in-a-lifetime experience," said another follower.