Mr Shimmin says he didn't speak with Tully while he was allegedly in the supermarket.
Later that day, as a massive manhunt was underway in the town and surrounding areas to try and find the killer gunman, Mr Shimmin saw a news item that said police were looking for a John Tully as a person of interest.
He went to his store manager, Devin Young to say that he believed Tully had been in their store earlier that day.
They reviewed CCTV footage together and Mr Shimmin picked out a person he identified as being Tully.
The footage was shown to the jury today, which shows a man wearing a dark green top entering the store and purchasing milk, chocolate, and a bag of lollies before leaving the shop and biking off.
Tully, 49, denies two charges of murdering Ms Noble and Ms Cleveland.
He's also accused of attempting to murder two fellow workers Lindy Louise Curtis and Kim Elizabeth Adams and other charges that include two counts of unlawful possession of firearms, and one of setting a man trap.
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".
Electrician Peter Sullivan had just picked up some supplies from Radcliffe Electrical in Ashburton's Moore St when he saw a masked gunman walking quickly towards the Winz office.
Both hands were on the gun, just above waist height, Mr Sullivan said.
He immediately parked his van on Moore St, still in sight of the Winz office, and phoned 111.
Mr Sullivan, who had a hearing aid, said he "didn't hear the bangs".
About 45 seconds after seeing the gunman, and while still on the phone to 111, he said he saw about four people running out of the office "running for their lives".
Moments later, he said, the gunman also exited the office to a bike which was chained to a 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, "You bloody bastard, you blew her to bits".
"That's when I realised things were really serious," Mr Sullivan said.
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.
Tully is not be represented in court by his own defence counsel. Instead is legally assisted two amicus, or friends of the court, lawyers Phil Shamy and James Rapley.
He is not present in court today. He was removed from the court yesterday after an angry outburst.
The trial, before Justice Cameron Mander, continues.
