Remutaka Hill has reopened in both directions after a three-car crash this morning.
Police were notified about the incident shortly after 9.40am.
Witness Jeff Ng was travelling north on the State Highway 2 road between Upper Hutt and Featherston.
He rounded a bend and saw smoke about a hundred metres away.
Ng said he walked towards a burning car and tried to use a small fire extinguisher, but the flames were too strong.
Another passer-by with a larger extinguisher said he had also tried to fight the blaze to no avail, he said.
Three people received minor to moderate injuries.