State Highway 1 is now expected to be closed until at least 8.00pm tonight.
The road has been closed since early this morning, after a truck carrying potentially hazardous chemicals rolled about 2km south of Pukerua Bay township about 3.45am.
Due to the dangerous materials spilled, the extraction of the vehicle and clearing of the lanes is a complex operation involving about 50 firefighters and support crew, including a hazmat unit.
The road is closed in both directions between Paekākāriki and Grays Road at Mana, with traffic being diverted over the Paekākāriki Hill Road, which is suitable for cars and other small vehicles, but not for heavy vehicles or campervans.
A convoy of trucks several kilometres long is backed up as far as Paraparaumu.
NZTA and Police have asked motorists to avoid the area. They realise that many people are likely to have plans to travel north today, but to consider their plans and alternate options.
The planned closure of Paekākāriki Hill Road to holiday traffic this afternoon has been cancelled.