A post mortem has confirmed that a man who died after an assault in the Far North on New Year's Eve suffered fatal head injuries from blunt force trauma.

Lee Rata, 35, died in Whangarei Hospital on New Year's Day. He was found on the side of Bonnett Rd with critical injuries at about 11pm on December 31.

Detective Inspector Kevin Burke said today that the team investigating Mr Rata's death had been boosted to 22 staff.

Five officers had travelled north from Waikato, Counties Manukau, Auckland and Waitemata districts to assist with the investigation.

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Mr Burke said the investigation team are now working on the inquiry to help with the large number of witnesses who need to be located and spoken to.

Mr Rata has been returned to his family, and his tangi is taking place in the Far North. He is expected to be buried tomorrow.

"We have received good assistance from the public which has helped us in establishing who were present at the function in Bonnett Road near where the assault occurred," said Mr Burke.

"This includes numerous images of people who were at the function. We are continuing to identify and speak with function guests and those who were nearby. A number of the guests have returned to their homes outside Northland and we would like these people to contact us.

"Our priority is to establish who was responsible for this vicious beating and hold them to account."

Police would like to hear from anyone who may have seen the victim earlier in the evening and people who were in the Bonnett Road area or attending the function to contact them on 09 408 6500.