Tonight will be the final night of disruptions to traffic through the Lyttelton Tunnel due to maintenance work.

The Canterbury NZTA has taken to Twitter to alert motorists to tonight's disruptions, which start at 9pm, with the State Highway 74 work due to be finished by 6am tomorrow.

They agency also urged motorists to take care on South Island roads today due to icy conditions.

The maintenance, which started on Monday night, has frustrated motorists and truckies using the city and South Island's economic lifeline, with overnight traffic the last few nights restricted to single-lane operation under escort, and a complete ban on vehicles transporting dangerous goods.

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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.

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

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, the NZTA said.

The agency also said at 9.41am that the Henley SH1 was again open to two lanes after this morning's crash near the intersection with Mclaren Gully Rd. The road was down to one lane due to the need to apply grit to combat the icy conditions after the 8.40am crash.

They urged motorists to take care in general today due to icy conditions on South Island roads, with the following specific warnings.

NEW: SH8 - Lindis Pass Summit - An ice warning has been issued. Drive carefully.

NEW: SH94 - Te Anau to Milford Sound - Black ice warning. Drive carefully.

NEW: SH1 - Henley, Mosgiel, Crash near the intersection with Mclaren Gully Rd - Road now open to two lanes.

NEW: SH1 - Shagpoint, Otago - Due to a crash the road is down to one lane. Care is needed.

NEW: SH85 - Kyeburn to Palmerston - Icy conditions exist and motorists are asked to slow down and drive to the conditions.

NEW: SH87 - Outram to Kyeburn - Icy conditions exist and motorists are asked to slow down and drive to the conditions.

The agency warned motorists that conditions are changing quickly, urging them to check the NZTA website for the latest conditions.