Police have arrested a person who was trespassing on the Southern train line between the Puhinui and Papatoetoe train stations in Auckland.

Emergency services were initially called to reports that a person had been hit by a train.

But Auckland Transport spokesman James Ireland said the man was unharmed.

"Police have found him. He's not been hit by a train."

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The train line was shut down in both directions but will be reopened shortly, Ireland said.