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Mr Andrews said there had been no obvious breach of hotel quarantine protocols, saying the infected worker was a "model employee".
Between 500 and 600 tennis players and officials in Melbourne for the Australian Open were told to isolate and get tested on Wednesday after a worker at one of the hotels they use tested positive for COVID-19.
All play was suspended at Melbourne Park on Thursday, with the tournament due to start in four days.
Thursday's play in the warm-up tournaments at Melbourne Park was cancelled after the case was announced late on Wednesday and the 520 who underwent quarantine at the Grand Hyatt hotel were instructed to get tested and isolate until they have a result.
Andrews told local media: "A 26-year-old man has tested positive for coronavirus".
"We are contacting all Australian Open players, officials and support staff who were staying at the Grand Hyatt during the entire period", said CQV, the government agency overseeing the quarantine of participants ahead of the Grand Slam. He worked his most recent shift last Friday.
Aggressive restrictions on incoming travel have helped keep the coronavirus at bay in Australia, making it one of the countries still able to have fans at sports events.
"We don't want to see a situation as we did with the Grand Prix, where crowds were literally turning up, only to be turned away", the Melbourne Herald Sun quoted O'Brien as saying.
Victorians will rightfully be furious if the virus spreads leading to further restrictions or lockdowns, given the controversary surrounding the Australian Open.
It's not yet known if the man is carrying a potent offshore variant of the virus.
The Department of Health and Human Services has listed exposure sites linked to the latest case.
The hotel advertises it has 550 rooms, including 25 premium suites, with potentially hundreds of people involved.
It could also test the resolve of players who have recently come out of a two-week quarantine and give ammunition to critics of the decision to allow people to fly in from all over the world for the Australian Open.
Some 100 hotel quarantine staff members and 37 returned travellers who have completed their 14 days on the impacted floor are now self-isolating at home.
Also on Thursday morning, the DHHS confirmed the infected hotel worker is a locally-acquired case, breaking the state's 28-day clean slate.
NSW Health on Thursday said it had begun screening passengers arriving on flights from Melbourne following the diagnosis.
Amid fears of further community transmission, Andrews announced the limit on at-home gatherings will drop from 30 to 15 as of midnight Wednesday.