"Firefighters are currently wearing breathing apparatuses and checking the contents, but there is believed to be no dangerous goods on board - it's precautionary only," Mr Radden said.
Police would not close the road completely to avoid major traffic disruptions to northbound commuters, he said.
KiwiRail spokeswoman Kimberley Brady said the derailed wagons were believed to be the last of 36 that were being pulled by the engine.
The crane was coming from Hamilton to remove the wagons.