"We believe the bag has fallen from a vehicle in the vicinity of St Andrew St and the Southern Motorway during an AOS callout to a domestic incident," she said.
At 8pm on Thursday, police were called to a Mosgiel house, where a 30-year-old man had barricaded himself inside with a running chainsaw. Officers spent six hours negotiating with the man, who was eventually arrested and charged with assault.
Insp Sparrow said the bag's contents, if not tampered with, posed no harm to the public. He urged anyone who found the bag to return it to the central Dunedin police station "no questions asked".
The aluminium canister had distinctive blue writing on it, and the five tear gas cartridges were "shotgun-like".
"An internal investigation is under way to determine exactly how the error occurred and to ensure it does not happen again," Insp Sparrow said.
Anyone with information about the lost items was asked to call Dunedin police on (03) 471-4800 or supply information anonymously through Crimestoppers on 0800-555-111.