A man critically injured trying to board a moving train has yet to be identified.

It is unclear whether the man fell or jumped from a flat-deck train carriage police say he was trying to ride on.

The man was taken to Middlemore Hospital and has since been moved to the intensive care ward at Auckland City Hospital.

Police say he tried to ride the moving train at Puhinui Rd Station, near Otahuhu, about 1.30pm yesterday.

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"He has received significant injuries and police are working to identify him," Inspector Tony Edwards said.

"The man is of solid build, wearing blue denim shorts, a white T-shirt, white running shoes, a blue cap and is believed to be of Maori or Pacific Island descent."

It remains unclear whether the man fell or jumped onto the carriage, but an investigation is under way.

Anyone who may know the man should contact the Manukau Police on (09) 261 1300.