A petition has started to increase the speed limit from 80km/h to 100km/h on Auckland's northwestern motorway.

The petition, started by Hamilton man, Bradley Scales, already has 4000 signatures on the Change.Org website. The target is 15,000 signatures.

"Why, when you have a brand new, four lane wide motorway, would you make the speed limit 80kmh? To 'improve safety'? That is the most ridiculous explanation I have heard.

"This is a state of the art motorway and features nice wide lanes, pull over bays in the instance of breakdowns and incidents, and brand new up to date LED lighting for at night," Scales says.

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The NZ Transport Agency has insisted the 80km/h speed limit is there for safety reasons.

An 80km/h limit was put in place on certain parts of the motorway when the Waterview Tunnel was officially opened on July 2.

The previous limit was 100km/h but as drivers heading towards the city pass the Patiki Rd offramp, overhead electronic signs remind those behind the wheel to drop their speeds down to 80km/h.

It is the same for those heading north, passing St Lukes, and drivers heading towards the tunnel from the Mt Roskill side.

Some motorists have since been questioning whether the new speed limits - outside of the tunnel - would change back. But authorities say people will have to get used to the change, as it is here to stay.

NZTA's system design manager, Brett Gliddon, said the new limit was based on international best practice.

"Worldwide best practice in road tunnels and approaches to them is to have an 80km/h speed limit to reduce the risk of a crash.

"The Waterview Tunnel interchanges have been designed and planned to be consistent with this worldwide practice."

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Gliddon said Auckland's motorways around the new tunnel - as well as through the central motorway junction - were complex systems with high traffic volumes and significant lane changes and merging.

NZTA continued to monitor operational and safety performance on that particular stretch of the motorway network and took on board customer feedback, he said.

"As with any new piece of infrastructure, we are monitoring the ongoing management of speeds through this area and whether the current 80km/h speed limit is the most appropriate to ensure safe and efficient journeys.''

Since opening, the Waterview Connection has had positive feedback overall.

There have been one or two minor incidents inside the tunnel.

On Friday morning, motorists heading south would have noticed the overhead electronic signs dropping the speed limit to 70km/h and 50km/h for those driving into the tunnel.

The NZTA reported that a truck broke down on the far left-lane of the tunnel about 8am.

Despite the incident, traffic continued to flow well.