The 71-year-old man was fatally stabbed at the Friendly Grocer Nightcliff Supermarket in Darwin on Wednesday after confronting an 18-year-old who was allegedly stealing from the shop.
The store owner asked the teenager to return the items, but the teenager allegedly stabbed him with an edged weapon before fleeing on a bicycle, Northern Territory police said.
Patrons provided first aid and emergency services carried out CPR, but the 71-year-old was pronounced dead at the scene.
The teenager has since been arrested by police but he is yet to be charged.
In a Facebook post, Northern Territory Chief Minister Lia Finocchiaro said her government's current focus was on the welfare of the man's family.
"It is devastating that we are at this point again," she said.
"Every Territorian knows that this is not okay and we reinforce our commitment that everything is on the table."
The death comes about two years after the murder of bottle shop worker Declan Laverty, who was killed while on shift at a BWS near Darwin airport.
His killing by a man who was out on bail prompted a tightening of laws under Ms Finocchiaro's government.
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