The Australian state of Victoria reported on Wednesday that a fresh cluster of coronavirus cases had grown to 15 in 3 days and that the coming 24 hours were particularly critical as some of those infected with the virus had visited crowded venues.

An infected person was among around 23,400 people who attended on Sunday an Australian Football League match, and officials have ordered thousands of fans to test and self-isolate. Supermarkets, cafes and pubs were also among the venues visited.

James Merlino, the state’s acting premier, noted that he would not rule out further curbs for the capital Melbourne, saying “these cases are linked and that’s a good thing, but we are very concerned by the number and by the kind of exposure sites.”

On Tuesday, protective face masks were made compulsory in hotels, restaurants and other indoor venues in the city of 5 million until June 4 and limits were placed on the number of people who could attend gatherings.

Of the 15 cases, 6 were newly reported on Wednesday. Australian authorities have identified 301 close contacts of the people infected and said that so far 80 have returned negative results.

The COVID cluster has been traced back to an Indian variant of the virus found in an overseas traveler who had completed quarantine, but officials have not worked out how it spread into the wider community.