Despite the NZ Transport Agency's announcement of a return to a 100km/h limit on the Northwestern Motorway around the new Waterview Tunnel, no date for it has been set.

The speed limit was lowered to 80km/h when the tunnel was built and a spokeswoman for NZTA said there was "quite a process" to go through to restore the higher speed limit.

She did not know if it would be completed this year.

NZTA guidelines set out a procedure for reviewing and changing speed limits involving multiple surveys to assess road conditions and roadside development.

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Additional physical infrastructure would need to be completed as well as the legal process, according to NZTA.

Several studies were completed before the speed limit was lowered to 80km/h when the new tunnel was opened.

System design manager Brett Gliddon said the decision to change to the higher speed limit was partially because of feedback from drivers.

"We've been monitoring the operational and safety performance on the state highways around the tunnel, as well as taking on board customer and stakeholder feedback."

Reports from these studies noted a high level of lane changing and large traffic volumes in the area, which led to the decision to lower the speed limit.

"The motorway system around the tunnel and through the central motorway junction is complex with higher traffic volumes and a significant amount of lane changing," Gliddon said.

"Speed limits need to reflect these conditions while also ensuring we keep motorists safe."

A petition started to raise the speed limit back to 100km/h received more than 13,000 signatures.

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