Cummins, who was eating in a restaurant but did not breach any biosecurity protocols, must isolate for seven days.

Australia lead the series 1-0.

"Gutted to miss this Test but really excited to see Neser finally get his chance in the baggy green," fast bowler Cummins, 28, said on social media.

"He has done the hard yards and is a seriously skilful player.

"Super frustrating but Covid has thrown us all some curve balls over the last couple of years. Will be cheering along!"

• Live: The Ashes - Australia v England

Cummins, who was eating out on Wednesday night, isolated as soon as he was made aware he was a close contact of the person who tested positive and has since had a PCR test, which was negative.

Fast bowler Mitchell Starc and off-spinner Nathan Lyon were eating at the same restaurant as Cummins but were at a separate table outdoors.

South Australia Health has deemed them casual contacts so they are free to play.

Cricket Australia (CA) expect Cummins to be available for the third Test at Melbourne, starting on 26 December.

Cummins was set to captain Australia for only the second time since replacing Tim Paine last month.

His absence is a further blow to Australia's fast bowling attack, with Josh Hazlewood already ruled out with a rib injury.

Smith, 32, will lead Australia for the first time since he was stripped of the captaincy for his part in the ball-tampering scandal and attempted cover-up in South Africa in March 2018.

The batter was banned for a year by CA and also barred from leadership positions for two years, before he was made vice-captain after Paine stepped down.

Former vice-captain David Warner was also banned for a year, while batter Cameron Bancroft received a nine-month suspension.

Smith has captained Australia in 34 of his 78 Tests so far, scoring 15 centuries at an average of 70.36 as skipper.

Batter Travis Head will be Australia's vice-captain for the second Test.

Neser, 31, has taken 228 wickets in 68 first-class matches at an average of 24.94.

According to the Australian Government Department of Health,

there are 92 active Covid-19 cases in South Australia currently, with 64 locally acquired in the last seven days.

Australia were due to be captained in this Ashes by Paine, who replaced Smith, but the wicketkeeper stepped down last month over a historical investigation into sexually explicit texts to a female colleague.

Paine subsequently withdrew from the series to take a break from cricket.

Australia beat England by nine wickets at the Gabba on Saturday to take a 1-0 lead in the five-Test series.

Analysis

Australian cricket commentator and journalist Geoff Lemon

Losing Cummins is a major blow, not so much because of his very new captaincy, but because it compounds the loss of Hazlewood to a side strain

Australia will now embark on a Test with Starc, Neser on debut, and Jhye Richardson in his first match in nearly three years.

An equally big part of the story is that Smith now returns as Test captain, even if temporarily.

There was discontent in many quarters about this when he was appointed permanent vice-captain a couple of weeks ago, as people anticipated the moment when Cummins would miss a Test. Nobody thought it would happen so soon, or in such strange circumstances.

But here is Smith, back again. CA chief executive Nick Hockley says he has served his ban and grown as a person. A good tranche of Australian cricket lovers, though, don't want to see this reputational laundering of a captain who was banned for allowing cheating to flourish in his team.

Hockley said the CA protocols have tried to strike a balance so that players aren't completely locked down.

However, he strongly implied he would ask players to eat outside at venues from now on. Which does rather raise the question of why that was not already the protocol, in a city like Adelaide where the virus is getting moving after a period of inactivity.