The ABC understands a person who attended the Golden Plains music festival south-west of Melbourne on the weekend has tested positive for COVID-19.
It is believed the person recently returned from overseas and became unwell on Sunday.
About 12,000 people bought tickets for the outdoor music and camping event on March 7, 8 and 9.
Headline act, The Pixies, have postponed the rest of their Australian tour.
Festival organisers sent out an email before the festival, encouraging people to clean their hands thoroughly and often, especially before eating and after going to the bathroom.
They also asked anyone with a viral or bacterial illness to stay at home.
Hand sanitiser was widely available at the festival.
The ABC has contacted festival organisers and the Victorian Health Department for comment.
-more to come