More than 70 people may have caught coronavirus as a result of the Green Man Festival near Crickhowell.
The four-day event was attended by thousands of festival goers with no social distancing restrictions while the country was under 'Level Zero'.
Public Health Wales is carrying out active public health surveillance around large events.
A spokesperson said: "Analysis of contact tracing data gathered by Test, Trace, Protect has identified 71 Welsh residents with confirmed coronavirus, where Green Man festival was reported as a possible exposure in the seven days before showing symptoms.
"However, it is important to note that these cases are not necessarily attributable to transmission at the festival, as Green Man would have been one of a number of exposures reported by cases prior to onset, including, for example: socialising outside the festival, travel to and from the festival etc."
Ticket holders were required to show either an NHS Covid pass or a recent negative lateral flow test to enter.
In a statement, Fiona Stewart from Greenman Festival said: "There was a lot riding on this and we didn't want to let anyone down.
"From our post-festival questionnaire, the overwhelming response was that those attending felt we had done all we could to protect them from Covid-19 and that they felt safe, which was something that mattered a great deal to us."
Ludlow Town Council told to fund parking order
Eight-figure sum quoted to fix Ledbury station access
Pothole chaos on Shropshire’s rural roads
Shropshire Council wants to link up with Telford and Wrekin
72 roads to be treated across Shropshire
Shropshire NHS called to act over cause of ‘twice as many deaths as breast cancer’
Pegasus FC cruise to huge win over Worcester Raiders in the Uhlsport Hellenic League Marsh Challenge Cup Semi Final
More than 550 new EV charging points coming
