Watch: Drone takes deep dive through new City Rail Link

New drone footage from Auckland's City Rail Link takes a dramatic dive from above surrounding hotel and office towers down around 14m below ground into the cavernous subterranean hole.

Work on the city end of the $4.4 billion project has been filmed from above and below in a new 1-minute film released by City Rail Link, the joint Government and Auckland Council venture.

In a clear illustration of the scope of New Zealand's largest transport project, the camera first focuses well above the hole between Britomart, the new $1b Commercial Bay office tower, H&M and the Mercure Hotel.

The camera then descends fast from between those buildings, hovers for a moment above the separate twin tunnels, then dives down deep inside them.

The drone moves from vertical to horizontal mode and then travels along the bottom of the hole towards the existing fully functioning Britomart train station.

Above, the facade of the historic former Chief Post Office is visible as the drone moves under that old structure.

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Partly completed tunnel wall, steel support beams, reinforcing rods and some of the relatively soft Waitemata sandstone can all be seen from the 20-second mark.

The Downer Soletanche Bachy Joint Venture has been on the site for about three years. having won the tender for Contract 1 at Britomart, CRL said.

"The JV team has recently completed the bulk excavation of two trenches under the CPO Building and Lower Queen Street, and is now working on the construction of the new train tunnels," CRL announced.