Despite the broken down truck being cleared from the road, traffic was still heavy coming in to Wellington city.

A single truck and trailer breakdown caused havoc for those commuting to Wellington from the Hutt Valley on Monday, leading to queues of up to 30 kilometres. .

An early morning breakdown on State Highway 2 south of Petone about 6.20am had a knock-on effect across the city.

One driver took an hour to get from the Haywards intersection to the Melling Bridge. Traffic along a huge section of SH2 at 9.15am was at a virtual standstill.

Drivers were taking alternative routes through the central Hutt to avoid SH2, causing major delays in Petone and on Rutherford St.

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The massive delays were triggered by a truck and trailer unit, which started to spew smoke and broke down on State Highway 2, just north of Petone, about 6.20am.

The truck and trailer was safely pulled over to the side of the motorway in Wellington.

Another vehicle breakdown on SH2 near Ngauranga soon after the truck was moved compounded issues.

Traffic at one stage was backed up to Maoribank in Upper Hutt - some 30km from the original crash - and was stretched back to the bays towards Eastbourne.

Much of Hutt City was also clogged as motorists inch their way to the motorway.

GOOGLE MAPS Google Maps showed traffic at 7.30am was backed up about 10km along SH2 from the breakdown near Petone.

As of 8.25am commuter Courtney Churchill said she had been in traffic for two hours.

About 9am commuter Phillip Chapman said: "The whole hutt Valley at a stand still. A normal three-minute trip taking almost an hour, just for a break down on [the] highway 10km away."

A witness to the truck breakdown said the vehicle was heading south when smoke started to billow from the vehicle.

"I was a few hundred metres behind it and could see what looked like smoke coming out of it. At first I thought it was some road works machine, but then everyone started hitting their brakes.

"The truck driver pulled over safely, but it was an awkward spot for traffic to get through. I made it past, but I know those single incidents can have a big ripple effect for the vehicles that come along later."

NZTA highway manager Neil Walker said when there was an incident on SH2, "given the network constraint of that route and significant peak flows in the morning and evening, there is the risk of congestion and delays when a natural event or incident happens."

Due to the size of the broken down truck, a specialist heavy haulage vehicle was required to remove it.

It arrived on site shortly after 10am, and the truck was cleared from the southbound lane just before 10.30am. It is usually the responsibility of the company that owns and operates the truck to arrange a tow truck.

Walker said future infrastructure such as Transmission Gully would help ease congestion by providing extra lanes.

But for SH2 the planned Petone-Grenada link road was one of the most important major infrastructure projects that would help resilience when an incident such as a crash, breakdown, flooding, storm or earthquake happened.

Walker said the agency was looking at optimising other forms of transport, such as rail and cycling. An investigation was underway to improve the cycling and walking connection between Wellington and the Hutt Valley which would enhance alternatives to driving.

DEBRIS CLOSES LANES, SPARKS HUGE JAM

Meanwhile emergency repairs on a patch of roadway in Auckland's Spaghetti Junction caused a 20km tailback for city-bound motorists on the southern motorway.

Police were called to the scene near the Nelson St off-ramp on the southern motorway about 6am on Monday.

The metal debris on the motorway had initially forced the closure of one lane between Wellington St and Curran St, police said.

NZTA said emergency road repairs would be carried out across two northbound lanes, leaving one lane open.

Northbound traffic on the southern motorway had severely congested between Spaghetti Junction and Papatoetoe to the south, leaving commuters facing a 90-minute drive.

However the jam had started to clear, and at 10.15am started to free up around Market Rd northbound.

Police said it was not known where the metal had come from and that there were no reports or visible signs of a crash.

However, the New Zealand Transport Agency earlier warned of a truck crash at Spaghetti Junction.