The Northwest Motorway between Rosebank Rd and town is now 80kmh.

Motorists looking forward to Auckland's Northwestern Motorway returning to 100kmh will be disappointed.

The speed limit is now 80kmh permanently between Rosebank Rd and Spaghetti Junction to "reduce confusion for drivers" and ensure safety, NZ Transport Agency's Brett Gliddon​ said in a press release on April 6.

The new 80kmh limit would also apply through the yet-to-open Waterview Tunnel all the way to the Dominion Rd interchange on the Southwestern Motorway.

NZTA/SUPPLIED The development of the Waterview Connection, pictured here under construction in May 2016, has led to a lower speed limit for about an 8 kilometre length of the Northwestern Motorway.

Before today, the Northwestern Motorway had a speed limit of 100kmh, but much of that has been reduced to 80kmh due to roadworks for years.

"The speed limit within the Waterview Tunnel will be 80kmh when it's opened so introducing a consistent speed limit on the motorway network around it, will reduce confusion for drivers," said Gliddon, the agency's Auckland highways manager.

The motorway approaches to the tunnel and the tunnel itself would also have overhead speed signs that could be changed.

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This would allow the speed limit to be reduced further in the case of an accident or maintenance work being carried out, Gliddon said.

New Zealand Automobile Association spokesman for Auckland transport issues, Barney Irvine, said the association was briefed on the speed limit changes – but did not endorse them.

"They felt a bit over the top to us," he said. "The distances involved did feel long."

It thought variable speed controls from the Newton Rd to Great North Road interchanges could be used to allow 100kmh speeds when the motorway was not too busy, he said.

The same stretch city-bound though "was a bit of a dog's breakfast".

"That road [heading to town] isn't really suited to driving at 100kmh because of the amount of merging and the fact that you've got traffic backed up where it's joining the Southern Motorway."

The Waterview Connection and tunnel had been planned to open in April 2017 but has been delayed a couple of months due to faulty sprinklers.

Gliddon said the new speed limits coming into effect before it opened would allow road users to adjust to the new limit and ensure their safety.

"We're making these changes now, ahead of the tunnel opening, to increase safety and give motorists time to get used to the new limits on this very busy section of the motorway."

The speed limit change was consistent with 80kmh over the Auckland Harbour Bridge and in the Central Motorway Junction, also known as Spaghetti Junction.