Moments later, Mr Sullivan said, the gunman also exited the office to a bike which was chained to a nearby stop sign.
Mr Sullivan said that "a Maori man" who had run out of the Winz office and across Cass St then challenged the gunman, shouting "You bloody bastard, you blew her to bits".
The man repeatedly challenged the gunman, the court heard.
When the alleged shooter started cycling away from the scene, the man then crossed Cass St and started chasing him eastwards down Moore St, getting to within about 5m of him.
"I supposed he was going to belt him up," Mr Sullivan said.
Tully denies two charges of murdering Work and Income employees Peggy Turuhira Noble, 67, and Susan Leigh Cleveland, 55, and attempting to murder Lindy Louise Curtis and Kim Elizabeth Adams. He also denies other charges that include two counts of unlawful possession of firearms, and one of setting a mantrap.
Identification of the shooter is key to the trial, the trial has heard.
The Crown says evidence that the gunman is Tully, and that he had an intention to kill, is "overwhelming".
Earlier today, distinctive black stickers featuring the letters "inX" that Tully allegedly used to label his property were "identical" to ones made by a local signwriter for a "tall ... well-built guy" just weeks before the slaying.
Signwriter Clive Burns Watson identified stickers on various items, including a broken down shotgun and shotgun cartridges, as being his work.
Mr Watson also identified an "inX" sticker found on a spent shotgun cartridge as being identical to the 120 he made for a man who came into his Ashburton store in mid-August 2014 wearing a dark black "almost German-style" helmet, a balaclava "or something very similar underneath that", heavy coat and fingerless woollen gloves.
In the Crown's opening address on Wednesday, Crown prosecutor Andrew McRae said a spent shotgun cartridge found inside the Cass St Work and Income office had a black "inX" sticker attached to it.
The trial, before Justice Cameron Mander, continues at 10am on Monday.
HOW THE TRIAL HAS UNFOLDED SO FAR
• Wednesday, Day One: The trial begins in the absence of the accused, Russell John Tully. The Crown outlines its case, saying Tully had been unhappy with the service he got from Ashburton Winz staff and entered the office at 9.51am on September 1, 2014, with the intention of shooting all of the staff dead. Identification of the shooter is key to the trial. But the Crown says it has "overwhelming" evidence that it is Tully.
• Thursday, Day Two: The jury gets its first glimpse of Tully, albeit briefly. After an angry outburst, where he claimed he is not fit to stand trial, Tully who is restrained in a wheelchair, is removed from the courtroom. Justice Cameron Mander instructs the jury to ignore the "pantomime" and to focus solely on the evidence. Winz assistant manager Jaimee Sarah Carrodus described Tully as a "very demanding ... manipulative" client who was "quietly intimidating to staff".
• Friday, Day Three: Tully not present in court. Ashburton signwriter identifies distinctive "inX" stickers that Tully allegedly labelled his possessions, including a shotgun and shotgun cartridges, as being "identical" to ones he earlier made for a man matching Tully's description. Electrician Peter Sullivan said he saw a masked gunman walking quickly towards the Winz office. About 45 seconds later, four people exit the office "running for their lives", later followed by the gunman. One of the fleeing men chases the gunman, yelling "You bloody bastard, you blew her to bits".
