Tunnel excavation under way on Mt Messenger bypass

Monday, 10 February 2025

Tunnelling has begun at the Te Ara o Te Ata – Mt Messenger Bypass project in North Taranaki.

Last week, the first cut was made by a 110-tonne road header machine that will excavate the project’s 235m tunnel.

Road headers have boom-mounted telescopic cutting heads, making them ideal for the diverse geological conditions that workers would encounter through the next stage of the project, NZ Transport Agency Waka Kotahi (NZTA) said in a statement.

A shovel plate at the front of the road header would collect the excavated rock and soil, which would then be conveyed along a belt to dump trucks at the back of the machine, it said.

The excavation work was expected to be completed later this year on a project which was last publicly estimated to cost $280m.

However, that cost is expected to significantly increase due to a number of factors, including inflation, complications in construction methodology, consenting requirements and six years of delays.

NZTA project manager Caleb Perry said a revised cost estimate for the project was due mid year.

The bypass aims to deliver improved safety and a more comfortable drive by avoiding the existing steep, narrow and windy route. It will include two bridges of about 125m and 30m in length, as well as the tunnel.

It is estimated it will save motorists between four and six minutes of drive time.

Perry said starting work with the road header was an exciting next step for the project.

“We’ll start to see some progress with the road header excavating up to three metres every day.

“The tunnel will be cut in two stages, with the upper portion [top heading] first, followed by the bottom section.”

Excavation would be regularly paused and a sprayed-on concrete applied to the crown and walls to line and support the structure.

“The design and construction of the tunnel are similar to the Northern Gateway Tunnel in Auckland and the tunnel will be large enough to accommodate loads up to and including house removals – this isn’t something that can currently be accommodated on the steep, narrow winding stretch of SH3,” Perry said.

When finished, the tunnel would “incorporate cultural elements acknowledging Ngāti Tama tūpuna, the traditional guardians of the northern gateway to Taranaki”, Perry said.

“Completion of the project is expected to take a minimum of four construction seasons from when the Mt Messenger Alliance starts work on the northern property.”

The existing State Highway 3 over Mt Messenger has one tunnel near the summit. It is just a handful of metres long and was originally just a single lane, dug by hand.

In 1983, a mechanical reamer was used to make the tunnel wide enough (just) to accommodate two lanes.