A 36-hour window used to identify how to 'future-proof” the Kaimai Tunnel for forecast freight train volumes will begin in the coming weeks.

KiwiRail. Photo: file.

The repair work starting September 13 is the first of its kind since the tunnel's opening in 1978, signalling a more direct freight link between Waikato and the Bay of Plenty.

KiwiRail senior communications advisor David Miller says the work – the start of a four-year $10m project – looks at identifying the upgrade requirements needed to future-proof the tunnel and meet forecasted freight volumes in years to come.

On average 153 services use the Kaimai Tunnel each week, including weekends, and this number is forecast to increase.

Contractors will be working for 36 hours in the tunnel from September 13, which is understood to coincide with a bridge on the Waikato side of the line being replaced.

Work will include using ground-penetrating radar scanning equipment to review the condition of the tunnel and installing drain and water flow monitoring systems.

General maintenance will also be undertaken, forming part of the work needed to cope with increasing volumes of freight, says David.

The condition of the tunnel was assessed last year by KiwiRail, with improvements to include installing a ventilation system.

Construction on the tunnel started in the west end in January 1969 before opening in 1978, becoming longest tunnel in New Zealand at 8.8km.

It is part of the Kaimai Deviation, which was constructed to bypass the old route of the East Coast Main Trunk Railway through the Karangahake Gorge.

The deviation is a 24km line between Waharoa and Apata, including several major cuttings, embankments and viaducts, while the tunnel makes up 8.85km of this distance 5km from the Apata end of the line.