At least 215 people had gastroenteritis after eating doughnuts from Mrs Kim's Donuts Kingston between November 20 and November 24, 2021, although it could have been more.

The shop remained closed from November until July this year, when it reopened under new management.

A Department of Health and Aged Care investigation report, released after a freedom of information request by The Canberra Times, said it was "was one of the largest foodborne outbreaks investigated in the ACT, [and] the true extent of illness remains unknown".

Most people reported diarrhoea, vomiting and abdominal pain, and symptoms generally lasted about 48 hours, although at least one person was sick for six days. Two people ended up in hospital. One of them had previously had gastric sleeve surgery, so was particularly susceptible.

In one case on the last day of the outbreak, 140 people attended a workplace morning tea where there were 192 Mrs Kim's doughnuts. Fifty-nine of those people contacted ACT Health and completed a survey, and 27 of them had gastro.

The department said the source of the contamination was unable to be confirmed, however "it is suspected that a food handler worked while infectious" and likely contaminated the doughnuts through cooking, decorating or serving them.

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Lab results of six food samples returned results for coliform, usually indicative of human faeces, in the doughnut fillings.

The shop's proprietor initially told ACT Health only two staff had worked in that period, however later revised this to say there had been two people preparing food and another two serving.

"The proprietor denied any reports of staff illness during the study period and reported no incidences of vomiting or diarrhoea on site among staff or patrons," the report said.

ACT Health epidemiologists hypothesised the symptoms were "suggestive of a viral infection, particularly norovirus".

"While all ill patrons reported eating a doughnut, no one flavour was identified at initial interview, suggesting that the source may have been due to a common ingredient, food handler and/or environmental contamination," the report said.

It said there were no reports of vomiting or diarrhoeal episodes among patrons onsite, "making it unlikely that environmental contamination from a patron was the source of infection".

"The food handlers at the food premises did not provide stool specimens for testing. These factors together suggest that an ill food handler engaging in food preparation was the most likely source of infection," the report said.

"There is the potential for a food handler to have worked with an asymptomatic norovirus infection; however, the scale of illness associated with this outbreak suggests there was a lapse in hand hygiene and proper food handling procedures regardless.

"While national food standards require exclusion from food handling of employees with a gastrointestinal illness, the lack of leave entitlements for casual employees may be a disincentive to remain away from work."

The report also noted the rise of third-party food delivery services was creating a challenge for authorities to investigate outbreaks, and the role of these services in outbreak investigations should be explored further.

ACT Health's disease surveillance team interviewed or surveyed 301 people and 71 per cent of respondents had gastro.

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