A cold case killer who shot a Turangi service station attendant to death at close range has died in custody.

Menzies John Hallett was convicted at the Rotorua High Court in 2013 of murdering Rodney Tahu.

The conviction came more than 30 years after the Turangi killing.

Department of Corrections said Hallett died on Wednesday.

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He "had been receiving treatment following a medical event" and died in hospital, the spokesperson said.

Police have been notified and have informed the man's next of kin.

All deaths in custody are referred to the coroner for investigation and determination of cause of death. For all deaths in custody, there is also an investigation by the independent Corrections Inspectorate, the spokesperson said.

Hallett was an inmate at Northland Region Corrections Facility at Ngawha.

Tahu, whose murder remained a cold case until late 2011, was found by a passing truck driver lying in a pool of blood on the forecourt of the Shell service station.

The 32-year-old married father of two sons died a few hours later at Taumarunui Hospital from gunshot wounds to the head and shoulder.

In May 2013, Hallett was found guilty of murdering Tahu 34 years earlier.

It was found that Tahu had locked up the service station, set the alarm and had just opened the door of his own vehicle to go home.

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Tahu told Hallett he was unable to help because the station was closed. Hallett took issue with this, claiming it was only three minutes past 1am, and called Tahu a "black b******".

As Tahu went to approach him, Hallett pulled a .22 calibre revolver pistol out of his belt and fired towards him, missing.

He then fired again hitting Tahu in the shoulder. As Tahu lay on the ground Hallett walked over, stood over him and shot a third time through Tahu's head. Tahu never regained consciousness and died at 5.42am in Taumarunui Hospital.

After shooting Tahu, Hallett drove on to Wellington where he told his wife, Susan, what he had done. He told her Tahu's refusal to help him at the service station had been the "flashpoint". He asked her not to go to the police for the sake of their daughters, however she did later that day.

However, due to the law at the time a wife couldn't give evidence against her husband.

A magistrate found there was insufficient evidence for Hallett to face trial for the murder, because all other admissible evidence was circumstantial. The file was eventually closed.

In 2006 the law changed so a wife could give evidence against her husband. The cold case was eventually reopened and Hallett was charged in late 2011.

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