UPDATED 3.20pm: State Highway 1, at Mercer, is now open after a train and container unit derailed this morning.

The Fire Service was called about 3.20am, after four wagons derailed on the north-bound train, with two obstructing the expressway on SH1 at Mercer.

File photo.

The special hazardous materials unit also attended as a precaution, but so far no dangerous substances have been found - only frozen cakes, cleaning products and confectionery. No one was injured in the incident.

NZ Transport Agency regional performance manager Karen Boyt says traffic flowed without any problems through a short diversion.

'The diversion for both directions of traffic through the north bound lanes ran welll and remained in place until the highway was reopend."