The New Zealand Transport Agency's overnight maintenance work in the Lyttelton Tunnel will continue for another week.

NZTA Journey Manager Lee Wright said the Agency had three different projects underway in the Lyttelton Tunnel this month.

These included the replacement of the centre line smart studs (illuminated cat's eyes) for the full length of the tunnel.

Contractors will also clean and replace the tunnel lighting, and install air-quality monitoring equipment.

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The work is essential, and will take place from 9pm to 6am, starting tonight and ending on Saturday morning, July 16.

Wright said by coordinating these works, the agency is saving a further seven nights of closure.

"Thanks to everyone for their patience and planning around these short delays while the one-way system operates this week."

The work will place restrictions on traffic flows through the tunnel and the movement of dangerous goods will not be permitted during this period.

Normal traffic will be restricted to single-lane escorted operation for the hours stated, with some delays expected.

Dangerous goods will only be allowed to pass through the tunnel between 7pm and 9pm, and between 6 and 7am.

Evans Pass is only due to reopen in late 2017 or early 2018. Photo / File
Evans Pass is only due to reopen in late 2017 or early 2018. Photo / File

Lyttelton Tunnel is a critical road link between the city and Lyttelton Port, originally designed to handle 2400 vehicles an hour and reduce the travel distance between the two by around 8km.

Since the February 2011 earthquake it has become even more of an economic lifeline for Canterbury and the wider South Island, critical for getting fuel supplies to Christchurch and beyond, and for exporters to get their produce to markets.

The long-term closure of Evans Pass because of rockfall damage means the tunnel remains the only viable freight access route to Lyttelton Port until the pass is due to reopen in late 2017 or early 2018, which will allow dangerous goods and over-dimensional vehicles to be rerouted, reducing the number of closures of the tunnel.