A chemical spill after a truck overturned near Pukerua Bay, north of Wellington, has closed SH1.

State Highway 1 between Plimmerton and Paekākāriki has finally re-opened to all traffic, after a mammoth 17-hour closure.

While New Zealand Transport Agency (NZTA) was yet to formally re-open the stretch of road to all vehicles affected by an early morning truck crash and chemical spill, around 400 trucks were cleared to move just after 9pm Friday.

The road was re-opened to all traffic shortly afterwards, police have confirmed.

"The waiting trucks, heading southbound were released through the cordon to clear a backlog at about 9pm this evening," a statement read.

"All traffic north and south is being allowed through, with stop-go traffic control in place. We thank motorists for their patience today."

NZTA systems manager Mark Owen said that while the road was accessible again, the road surface was damaged in the crash.

For that reason, motorists were being warned there may be delays. More repair work would be carried out on Saturday.

"Cleaning up the truck and hazardous materials that were spilled in the crash hasn't been straight-forward and we hope to have both lanes through site operating once the material has been removed and the road surface repaired," Owen said.

Fire and Emergency officials worked all day to remove a variety of dangerous chemicals which spilled when a truck crashed north of Wellington at 3.45am Friday.

Trucks were backed up for kilometres since the early morning, with no way out of the gridlock.

The chemical-laden Mainfreight truck was carrying mainly paint products when it crashed, however the truck's driver escaped with minor injuries.

The clean-up efforts were complicated by the mixing of a number of different chemicals, forcing the road to be closed to all traffic.

ROSA WOODS/STUFF Parked trucks backed up on State Highway 1 near Paraparaumu on Friday.

A Fire and Emergency spokesperson said a definitive list of the chemicals had not yet been compiled, but there were "a lot of them".

The spokesperson did not want to speculate on what some of those chemicals could be.

Staff were working to remove the dangerous chemicals from the truck-and-trailer unit by hand, which made the clean-up a "long, slow process".

NZTA said, because of the nature of the chemicals, clearing the vehicle and the lanes was a "complex operation".

SUPPLIED The two trucks have become stuck on a corner on Rimutaka Hill Rd.

A convoy of trucks several kilometres long is waiting for SH1 to reopen, just as motorists prepare to exit Wellington for the long weekend.

At one point both SH1 and SH2 were closed on Friday. Light vehicles can take the hilly, windy Paekākariki Hill Rd, but trucks can't.

NZTA regional transport systems manager Mark Owen said: "If people are heading out of town, they need to think of an alternative at this stage, and it's certainly going to take a lot longer if they take an alternative route."

Booth's Transport truck driver Max Wyllie has been stuck in an unmoving line of trucks and buses since about 5.30am, which he said spans as far back as the Paraparaumu exit, a 20km distance.

KYLE MITCHELL/SUPPLIED Fire & Emergency staff test for hazardous chemicals spilled in the SH1 accident.

"There's maybe a dozen refrigerated trucks along the bay here ... they have their machines going on separate batteries, and can last up to about 10 hours."

Wyllie said passengers on the buses stopped in traffic hopped off and walked towards the Paekākāriki train station after about two hours of going nowhere.

"Most of the drivers that have been here since 4am or 5am have to catch ferries."

SUPPLIED A truck carrying chemicals crashed, blocking both lanes of State Highway 1, north of Wellington.

He said some drivers have been ringing their depot about going over the legal limit of consecutive hours, which is 14 hours, and were told not to log the hours they sit in traffic, meaning some would not get paid for the hours they spent stuck there.

A Greater Wellington Regional Council spokesman said a duty officer was on site to assess the environmental impact of the chemical spill, but it was not yet known whether the spill will impact waterways.

Heavy vehicles were unable to use SH1 Plimmerton to Paekakariki until further notice and were being asked to avoid the area, however light vehicles were still able to access the road.

SUPPLIED The truck rolled on State Highway 1 in Pukerua Bay at about 3.45am Friday morning.