A man who fell out of a moving car in Porirua has died and police say they are treating his death as a homicide.

Detective Senior Sergeant Christiaan Barnard said 19-year-old Aucklander Sosiua Helotu Ula fell from a moving vehicle on Warspite Ave about 4pm on Sunday.

He was found lying on the road with critical injuries.

Ula, who is from Wiri, died in Wellington Hospital last night.

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Barnard said police were still seeking the public's help identifying anyone who may have witnessed the incident on Sunday.

He said police were trying to reconstruct Ula's movements before he sustained his fatal injuries.

Ula was described as solidly built, around 185cm tall, with a short black hair cut. He had a small amount of facial hair on his chin.

Barnard said police wanted to hear from anyone who saw a black Mercedes, registration GPW495, or any other car driving erratically near Waitangirua Mall on Sunday.

Police also wanted to speak to the driver of a red car driving east on Warspite Ave that nearly had a head on crash with the Mercedes, he said.

One person had been arrested in relation to the incident and charged with driving while disqualified and supplying methamphetamine.

If you have information call the Wellington Police Crime Squad on (04) 381 2000 or anonymously call Crimestoppers on 0800 555 111.