The largest transport infrastructure project New Zealand has ever built will unlikely open to the public until 2026.
Auckland's City Rail Link's boss said it could take months after construction, which is due to finish at the end of 2025, has been completed until passengers can ride the trains.
The twin tunnels below Auckland's city centre will create a 3.5km link between Britomart and Mt Eden, at peak times, transporting an estimated 54,000 passengers an hour.
Originally set to finish in 2024, the multi-billion dollar project has been plagued by disruptions since work started in 2016.
In March, CRL moved the completion date, estimating construction to be done by November 2025 and the cost estimation blew out by just over $1 billion to a total of just under $5.5 billion.
But passengers won't be able to ride the trains until months after completion.
"Once built and handed over to KiwiRail and Auckland Transport, the new systems need to be linked into the existing older systems, with significant testing required," A CRL spokesperson said.
These processes include testing the trains and infrastructure, training drivers, new safety procedures and linking the new systems into the old.
"Overseas experience shows the linking of new and old technology is a major risk to the project and will take time," the spokesperson said.
But just how much time is unknown.
Speaking to the NZ Herald, City Rail Link chief executive Dr Sean Sweeney said the testing could take six months.
"As a ballpark guess, I'd say six months, but people need to understand that numbers could change a lot, based on what happens," Dr Sweeney said.
An Auckland Transport spokesperson told Newshub they are working with key stakeholders on a plan that enables passenger trains to start operating as early as possible.
"This plan allows time for AT, Auckland One Rail (AOR) and KiwiRail to complete critical testing of the new systems before trains can operate, including emergency and evacuation protocols, driver training, signalling and other important systems needed to operate CRL safely," the spokesperson said.
"We will be able to confirm the dates for CRL operations once this programme is complete."
Prime Minister Chris Hipkins said Aucklanders will want to see the trains opened as quickly as they can but there have been a lot of disruptions and cost escalations to the project.
"I acknowledge the frustration that the Auckland community will be feeling, they just want to see this done, we just want to see it done as well," Hipkins told AM co-host Ryan Bridge on Tuesday.
"I am confident that the CRL knows that we want to see it done and they'll be working as quickly as they can in order to do that but, ultimately, they can only go as fast as they can go."