The next stage of the construction of Auckland's City Rail Link is underway.
Concrete's being poured to build the rail tunnel boxes underneath Britomart's historic, heritage-listed Chief Post Office building.
It is at the bottom of Queen Street and serves as the entrance to the train station.
It follows more than two years of work to reinforce the building before anything could go underneath it.
Seventy cubic metres of concrete is being poured to build the first-floor section of the northbound tunnel.
City Rail Link chief executive Sean Sweeney said it had been an extremely challenging part of the project.
"Our teams have been working in some pretty demanding conditions, not only in reclaimed land immediately adjacent to a live railway station, but also excavating below sea level with the Waitemata Harbour only a few metres away.
"On top of that, some remarkable skill and innovation has been used to protect the Chief Post Office building so that we could get underneath it."
He said they had been working towards this day since July 2016.
Mr Sweeney said the pour was an important moment for the project and its contractor, Downer Soletanche Bachy.
Once the rail tunnel boxes are done later this year, it will be possible to walk about 400 metres of the route underground, from Britomart, up Albert Street as far as Wyndham Street.
The 3.45 kilometre-long train line, which at this stage is projected to cost $3.4 billion, will be completed in 2024.