The state government and Cedar Meats' management have defended their handling of the outbreak, including the decision to allow staff to work for several days after workers tested positive.
Cedar Meats general manager Tony Kairouz says he has been advised by WorkSafe of its investigation.
"We welcome it. We will co-operate fully," Mr Kairouz said in a statement to AAP on Wednesday.
It comes as another cluster continues to grow in the city's north.
Three more infections have been linked to McDonald's Fawkner, bringing the total number of cases linked to the fast-food restaurant to six.
That number could rise as test results for 92 workers continue to come back.
The restaurant has reopened following a deep clean and is being staffed by workers from surrounding outlets, a McDonald's spokesperson said.
Meanwhile, a worker at a poultry processor has also tested positive to the virus.
Health authorities did not name the facility but said it was an isolated case. The business is not an abattoir and contact tracing is under way.
Victoria's coronavirus count stands at 1514, with seven new cases confirmed on Wednesday.
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