Australia captain Pat Cummins has been ruled out of the second Ashes test in Adelaide later on Thursday after being identified as a close contact of a COVID-19 case, Cricket Australia said.

The board said Cummins would need to isolate for seven days and would look to play in the third test in Melbourne from December 26.

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Cricket Australia confirmed media reports that the fast bowler was dining at an Adelaide restaurant on Wednesday night where a positive COVID-19 case was identified.

"(He) did not breach any biosecurity protocols," CA said in a statement.

"He isolated as soon as he became aware of the situation and has since had a PCR test, which produced a negative result.

"SA Health has confirmed that Cummins is a close contact and will be required to isolate for seven days."

Cummins, the world's top-ranked test bowler, led Australia to a nine-wicket win over England in the Ashes opener in Brisbane, his first match as captain.

His exclusion from the Adelaide test is a blow for Australia as they look to take a 2-0 lead in the five-test series.

Cricket Australia said fast bowler Mitchell Starc and spinner Nathan Lyon were dining at the same restaurant as Cummins but at a separate table outdoors.

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They were deemed casual contacts and are free to play.

Vice-captain Steve Smith will take over the captaincy in Cummins' place, regaining the role nearly four years after losing it for his involvement in the 2018 Newlands ball-tampering scandal.

Travis Head will be vice-captain, while uncapped paceman Michael Neser will replace Cummins in the attack.

"Cummins is understandably very disappointed not to be able to captain Australia for the day-night test in Adelaide," CA said.