Auckland's Southern line train services are facing lengthy delays after a truck hit a train overbridge overnight.

Passengers were caught up in the chaos early this morning - around 7am - when Auckland Transport suspended all services on the route due to an earlier crash in Ellerslie.

An update just before 7.30am said Southern line train services were back on after engineers cleared the bridge.

However, trains would only be able to travel on it on a reduced speed.

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"Please expect minor delays as we resume scheduled service.''

A police spokeswoman said they received a report about 4am that a truck had hit an overbridge on Walpole St.

A police unit was sent about 7am to check whether the truck was still stuck under the bridge.

Officers reported the truck was no longer there and that the underpass had minor damage.

It is understood the person who made the report to police early this morning was connected to the incident.

Police said the incident was referred to KiwiRail, the Auckland Transport operations centre and NZ Transport Agency.

Some services, however, have already been affected and passengers at some stations - including Britomart - were transferred to buses.

A passenger who contacted the Herald said he was due to catch a train at 6.40am at Britomart.

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"It's just chaos here,'' he said.

There were no overheard signs or announcements to advise passengers at Britomart of the situation, he said.

People were just being redirected to buses.

"There's been no information whatsoever,'' he said just before 7am.