Michael Sulivan, who lived nearby the station, had shared a cup of tea of Mr Tahu hours earlier and was one of the first to find his friend bleeding on the forecourt.
He found him lying on the ground, with "extensive frontal injuries" and bleeding "profusely from his mouth".
"There was a large pool of blood from his mouth on the concrete," he told the court.
Unable to find a phone at the station to call for help, he ran 50m to his home and called from there.
A doctor was brought to the scene, before Mr Tahu was taken by ambulance to Taumarunui Hospital, where he died a few hours later.
Ronald Ensor, who was a pathologist in Rotorua at the time, also gave evidence this morning.
Mr Ensor described how the fatal shot had travelled through the left side of Mr Tahu's brain and richocheted off the inside of his skull.
There was no way Mr Tahu could have recovered from the trauma, he told the court.
Jurors were earlier told Hallett had called Mr Tahu a "black bastard" when he was told he could not receive oil for his vehicle.
When Mr Tahu took a few steps toward him, Hallett drew his .22 revolver and fired at Mr Tahu three times - once missing, once striking him in the shoulder as he tried to run, and the third striking his head as Hallett allegedly stood over him and took aim.
The court heard that, a few hours later, Hallett told his wife in Wellington: "I've killed someone. Yes I have."
She soon informed police, who waited for Hallett to return to his home in Taupo and eventually arrested him after a stand-off on the Napier-Taupo Rd, in which he injured himself with a shotgun round.
But the original murder case against Hallett failed because of a lack of evidence, with laws at the time stopping his estranged wife from giving evidence.
With those restrictions since lifted from a change in the Evidence Act 2006, Susan Sharpe is now considered the Crown's key witness in the new trial.
Jurors have been told they will not be asked to decide whether to acquit Hallett, but to choose between convicting him of murder or manslaughter.
Hallett's defence counsel, Paul Mabey, QC, earlier told them there was an admission that Hallett caused the death of Mr Tahu.
"What you will focus on at the end of the trial... will be whether your verdict is murder or manslaughter."