England's preparations for the fourth Ashes Test fell victim to more Covid chaos today, with local net bowlers removed from their latest training session on the same day head coach Chris Silverwood joined the growing ranks of positive cases.

Silverwood has been isolating with his family in Melbourne ever since one of them tested positive after the St Stephen's Day Test and a further case in their group takes the total number among the wider England party to nine.

He was already resigned to missing the new year game in Sydney, but his diagnosis, without symptoms, settles the issue. He will now aim to rejoin and lead the squad for the series finale in Hobart later this month.

Silverwood is the fourth member of coaching staff to contract the virus, joining bowling lead Jon Lewis, spin mentor Jeetan Patel and strength and conditioning specialist Darren Veness.

That left Silverwood's assistant Graham Thorpe to lead a threadbare backroom team in the squad's first practice session at the SCG, alongside temporary consultants Ant Botha and James Foster, and it was not long before that began to go off the rails too.

As is standard, England's batters were being put through their paces by club cricketers from the surrounding area - a widely-used policy that allows lengthier sessions without imposing unreasonable workloads on the Test bowlers. But it soon became clear that there was a problem, with the net bowlers asked to leave after around 40 minutes.

It soon transpired that one of them had returned a positive test and, although there was some suggestion that his result bore the hallmarks of a past infection, continuing the session would have compromised things even further. Questions are now being asked about how robust Cricket Australia's vetting procedure has been.

In the absence of bowling reserves England persevered without the cavalry, with captain Joe Root at one stage seen helping out with the side-arm 'dog stick' - a tool coaches use to artificially recreate high pace bowling for long periods.

Elsewhere, other members of the extended support staff such as communications and medical staff chipped in, helped perform drills and operate training equipment.

Former one-day captain Adam Hollioake had been drafted in to bolster a threadbare backroom team but, in keeping with an increasingly luckless trip, he was identified as a close contact after driving more than 500 miles from his home on the Gold Coast.

He will now isolate at the team hotel without touching base with the squad.

An England spokesperson said: "As soon as we became aware of the positive test, net bowlers were removed from supporting our practice. Practice remained in place until we finished at 1.00pm and that positive case did not cancel practice."

Another round of daily PCR testing was next on the agenda, the squad's seventh in eight days since seven positives in their wider travelling party were recorded.

Only one player on either side has so far tested positive - Australia's Travis Head - but there remains a high sense of alert about further spread inside either camp.

England opener Zak Crawley said he was happy to continue as scheduled, even if that meant tighter bubble restrictions.

"Personally I would (accept that), we haven't got long left. I can't speak for everyone but I want to play the two Tests," he said.

"We've spoken to the doctors, they've given us the facts and are more than happy to carry on. We fully trust that and I 100% want to play this Test, I'm definitely comfortable. We've managed to catch the cases early so we've been lucky not to have any players go down."

Externally, cases in Sydney continue to soar and a number have also been identified around the Test, including match referee David Boon and several members of the media.

Glenn McGrath, who has hosted a breast cancer fundraiser and awareness campaign during the new year Test for the past 13 years, in honour of his late wife Jane, has had to withdraw from the traditional launch event.

He is hoping to attend on day three, when the SCG is decked out in pink for the McGrath Foundation, but is currently in isolation following a positive result.

Meanwhile, the Big Bash League continues to go ahead despite increasing numbers of players being forced to withdraw, with grade players drafted up into the playing XIs to keep the show on the road.