A man found lying on a Whanganui street with unexplained injuries has died.

Police have launched a homicide investigation in the death of the 56-year-old who was found unconscious and seriously hurt last month.

The man was discovered lying in Tinirau Street in Whanganui East in the early hours of Friday, April 20. He was immediately taken to Wellington Hospital with critical head injuries.

This week he was transferred to Taranaki Base Hospital in what was described as a "serious condition". He died from his injuries on Wednesday night.

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Mystery still surrounds how he got his injuries and whether he was the victim of an attack.

Whanganui East resident Cara Davis told the Chronicle she was going home from Whanganui Hospital about 3am on April 20, and discovered the man on the road.

"I came round the corner into Tinirau St and saw a guy lying on the road unconscious," Davis said.

"I rang the police and ambulance but I was by myself so I didn't want to get out of the car.

"The police turned up pretty much straight away. They couldn't wake him and took him away in the ambulance.

"It was pretty heart-wrenching ... I'm still shaken up about it. I hope they can find out something about it."

Davis said the man was lying at the Duncan Street roundabout end of Tinirau St.

Police want to hear from anyone who saw or heard anything out of the ordinary in the area in the early hours of the morning on April 20.