Almost 30 people could have been infected with E coli bacteria after petting animals at the Ekka show in Brisbane.
So far, 12 people are confirmed to have been infected and 17 others are being tested.
Of those infected, four have been hospitalised and later discharged.
Chief Health Officer Jeannette Young says she's confident the outbreak, which came to light last week, can be controlled.
She said anyone displaying symptoms of Shiga toxin-producing E coli (STEC), including bloody and persistent diarrhoea, should quarantine themselves from others until symptoms cease.
"We are particularly urging parents to take their children to their GP for testing if they display symptoms of STEC," Dr Young said.
"If their doctor believes they may have STEC they should not attend child care or school until testing shows they do not have STEC."
The outbreak first emerged after a woman and her three children tested positive.
Authorities tracked the source down to the petting zoo at the Royal Brisbane Show, or Ekka, and others who attended have since come forward.
STEC is a serious, infectious type of bacteria and can spread easily from person to person without proper hygienic practices.
In rare cases it can develop into a haemolytic-uraemic syndrome, which affects the kidneys, and in some cases can be fatal.